Process and apparatus for manufacturing pure forms of aromatic carboxylic acids

ABSTRACT

A process and apparatus for manufacture of aromatic carboxylic acids comprises a liquid phase oxidation of aromatic hydrocarbon feed materials and treatment of a high pressure off-gas from the liquid phase oxidation to separate water and reaction solvent and purification of impure aromatic carboxylic acid products wherein a purification liquid includes water from off-gas treatment.

This application is a continuation of U.S. application Ser. No.11/909,117, filed Sep. 19, 2007, now U.S. Pat. No. 8,173,834.

FIELD OF THE INVENTION

This invention relates to a process and apparatus for manufacture ofpure forms of aromatic carboxylic acids by oxidizing aromatic feedmaterial to an impure, crude product in a liquid phase reaction mixtureand purifying impure forms of aromatic carboxylic acid, with treatmentof off-gas from liquid phase oxidation to recover a liquid comprisingwater and use of liquid comprising water recovered from oxidationoff-gas in purifying impure aromatic carboxylic acid.

BACKGROUND OF THE INVENTION

Terephthalic acid and other aromatic carboxylic acids are widely used inmanufacture of polyesters, commonly by reaction with componentscomprising ethylene glycol, higher alkylene glycols or combinationsthereof, for conversion to fiber, film, containers, bottles and otherpackaging materials, and molded articles.

In commercial practice, aromatic carboxylic acids are commonly made byliquid phase oxidation in an aqueous acetic acid solvent ofmethyl-substituted benzene and naphthalene feedstocks, in which thepositions of the methyl substituents correspond to the positions ofcarboxyl groups in the desired aromatic carboxylic acid product, withair or another source of oxygen, which is normally gaseous, in thepresence of a bromine-promoted catalyst comprising cobalt and manganese.The oxidation is exothermic and yields aromatic carboxylic acid togetherwith by-products, including partial or intermediate oxidation productsof the aromatic feedstock, and acetic acid reaction products, such asmethanol, methyl acetate, and methyl bromide. Water is also generated asa by-product. Aromatic carboxylic acid, typically accompanied byoxidation by-products of the feedstock are commonly formed dissolved oras suspended solids in the liquid phase reaction mixture and arecommonly recovered by crystallization and solid-liquid separationtechniques. The exothermic oxidation reaction is commonly conducted in asuitable reaction vessel at elevated temperature and pressure. A liquidphase reaction mixture is maintained in the vessel and a vapor phaseformed as a result of the exothermic oxidation is evaporated from theliquid phase and removed from the reactor to control reactiontemperature. The vapor phase comprises water vapor, vaporized aceticacid reaction solvent, oxygen gas not consumed in oxidation, gaseousby-products such as methanol, methyl bromide and methyl acetate, carbonoxides and, when the oxygen source for the process is air or anotheroxygen-containing gaseous mixture, nitrogen, carbon oxides and otherinert gaseous components of the source gas.

Pure forms of aromatic carboxylic acids are often favored formanufacture of polyesters for important applications, such as fibers andbottles, because impurities, such as by-products generated from aromaticfeedstocks in such oxidation processes and more generally, variouscarbonyl-substituted aromatic species, are known to cause or correlatewith color formation in polyesters made from the acids and, in turn, offcolor in polyester converted products. Pure forms of aromatic carboxylicacids with reduced levels of impurities can be made by further oxidizingcrude products from liquid phase oxidation as described above but at oneor more, progressively lower temperatures and oxygen levels, and duringcrystallization to recover products of the oxidation, for conversion offeedstock partial oxidation products to the desired acid product, asknown from U.S. Pat. Nos. 4,877,900, 4,772,748 and 4,286,101. Pure formsof terephthalic acid and other aromatic carboxylic acids with lowerimpurities contents, such as purified terephthalic acid or “PTA”, aremade by catalytically hydrogenating less pure forms of the acids, suchas crude product comprising aromatic carboxylic acid and by-productsgenerated by liquid phase oxidation of aromatic feedstock, in solutionat elevated temperature and pressure using a noble metal catalyst. Incommercial practice, liquid phase oxidation of alkyl aromatic feedmaterials to crude aromatic carboxylic acid and purification of thecrude product are often conducted in continuous integrated processes inwhich crude product from liquid phase oxidation is used as startingmaterial for purification.

The high temperature and pressure vapor phase generated by liquid phaseoxidation in such processes is a potentially valuable source ofrecoverable acetic acid reaction solvent, unreacted feed material,reaction by-products and energy; however, its substantial water content,high temperature and pressure and corrosive nature due to componentssuch as gaseous methyl bromide, acetic acid solvent and water posetechnical and economic challenges to separating or recovering componentsof the off-gas for recycle and recovering its energy content. Further,even minor amounts of impurities that remain unseparated in recoveredprocess streams can prevent re-use of the recovered streams ifimpurities adversely affect other process aspects or product quality. Asdescribed in U.S. Pat. No. 5,200,557, for example, monocarboxylic acidsadversely affect hydrogenation catalysts used in purification processes,with even low levels of acetic acid residues such as present in crudearomatic carboxylic acid products recovered from oxidation reactionliquids being considered detrimental.

British Patent Specification 1,373,230, U.S. Pat. Nos. 5,304,676;5,723,656; 6,143,925; 6,504,051, European Patent Specification 0 498 591B1 and International Application WO 97/27168 describe processes formanufacture of aromatic carboxylic acids by liquid phase oxidation ofaromatic feed materials in which a high pressure off-gas is removed fromthe oxidation and treated for recovery and recycle of portions orcomponents thereof and, in some cases, recovery of energy. Waterrecovered from high pressure oxidation off-gases in these processes isgenerally returned to oxidation with acetic acid condensed fromoff-gases, recycled to off-gas separations for use as reflux or disposedof in liquid purge streams. Water condensed from oxidation off-gas andthen purified by distillation is used to wash a purified terephthalicacid precipitate according to embodiments of U.S. Pat. No. 5,304,676 andtreated water condensed from an expanded, low pressure gas afterseparation of monocarboxylic acid and water vapors in a high pressureoxidation off-gas, catalytic oxidation of a resulting high pressure gascomprising water vapor to remove organic impurities by conversion towater and carbon oxides and expansion of the resulting gas to recoverenergy is used as a crystallization solvent for purified terephthalicacid according to embodiments of U.S. Pat. No. 5,723,656. However, noneof the processes uses liquid condensed from a high pressure off-gas froma liquid phase oxidation as solvent or other liquid comprising water inthe purification of impure aromatic carboxylic acids. Further,recoveries of materials and energy from off-gases in such processesoften are accomplished at the expense of each other, for example due toloss of energy content on cooling or depressurizing to recovermaterials, burning of materials to control atmospheric emissions andother losses of oxidation solvent, feedstock and intermediates thatresult if the high temperature and pressure vapor phase resulting fromoxidation is not cooled or depressurized for removal of such materials.Impurities remaining in recycle streams can upset process operation andimpair product quality. Added equipment and process steps for recoveringmaterials, energy or both can add further process complexities and limitor preclude their practical utility if they add costs that outweighmaterials and energy savings.

Impact of lost energy and materials are magnified by scale of processoperations. In world-scale commercial manufacturing plants with annualcapacities of 500,000 to 1,000,000 or more tons of product, evenfractional percentages or hundreds of parts per million of feedstock andsolvent lost or converted to undesired or unusable by-products, minorinefficiencies in energy recovery and incremental additions to effluentwater treatment translate to significant practical losses of materials,increases in consumption of fuel or electricity and added processing, aswell as unpredictable process efficiencies and economics due todifferences and variations in costs for energy, materials andrequirements for treatment of gaseous and liquid emissions andeffluents.

SUMMARY OF THE INVENTION

This invention provides a process and apparatus which provide or enablemanufacture of pure forms of aromatic carboxylic acids with improvedrecovery and re-use of materials, recovery of energy or both and, inparticular, recovery of condensate liquid comprising water from a highpressure off-gas from liquid phase oxidation of aromatic feed materialsfor use in purification of aromatic carboxylic acid product from liquidphase oxidation or other impure forms of aromatic carboxylic acid.

Among the important features of the invention is the finding that a highpressure vapor phase generated as off-gas or overhead in the liquidphase oxidation of aromatic feed materials to aromatic carboxylic acidscan be treated for substantial removal of reaction solvent therefrom ina liquid stream, while a high pressure gas resulting from the separationcomprising steam and usually also containing one or more ofincondensable components of the oxidation reaction off-gas, unreactedaromatic feed material and reaction by-products, and at leastsubstantially free of solvent monocarboxylic acid, can be processedpractically and efficiently at high pressure to recover a liquid whichcomprises water and is substantially free of organic impurities from theoxidation, such as monocarboxylic acid solvent and its reaction productsfrom oxidation, and is suitable, without need for additional treatment,for use as solvent or other process liquid comprising water in processesfor purification of impure aromatic carboxylic acids. Condensate liquidrecovered from a high pressure gaseous overhead stream resulting fromseparation of a high pressure vapor phase from liquid phase oxidationinto solvent monocarboxylic acid-rich liquid and a high pressure gascomprising water can replace in whole or in part demineralized or otherpurified water sources used in known purification processes. One or moreof reaction feedstock and by-products from liquid phase oxidation,including by-products of the aromatic feedstock material and of themonocarboxylic acid solvent for oxidation, may also be recoveredaccording to embodiments of the invention. The high pressure vapor phasefrom liquid phase oxidation also represents a source of recoverableenergy and treatment of that vapor phase according to embodiments of theinvention allows for the recovery of energy therefrom or frompressurized gases derived therefrom. Energy can be recovered in the formof heat, in the form of work or as both. In some of its embodiments, inaddition to providing liquid comprising water suitable for use inpurification, for example as solvents for a purification reactionsolution or crystallization or recrystallization in recovery of pureforms of product or as wash or seal flush liquids, and the otherbenefits noted above, the invention can afford surprising flexibilityfor making a range of pure forms of aromatic carboxylic acids.

In integrated processes for manufacture of pure forms of aromaticcarboxylic acids comprising oxidizing aromatic feed material in a liquidphase reaction mixture to crude product comprising aromatic carboxylicacid and oxidation by-products of the feed material, and purifying thecrude product by hydrogenation of a solution thereof in a liquidcomprising water, the invention can eliminate or reduce requirements fordemineralized water or pure water from other sources and provide balancebetween water generated in the liquid phase oxidation and water consumedin purification not achieved in, and substantially improved over, knownprocesses.

The liquid comprising water that is recovered from the high pressurevapor phase resulting from liquid phase oxidation of aromatic feedmaterials to aromatic carboxylic acids and used in purification ofimpure aromatic carboxylic acid according to the invention is recoveredfrom a pressurized gas remaining after substantial separation of solventmonocarboxylic acid and water in a high pressure off-gas from liquidphase oxidation by condensation from the pressurized gas, preferablysuch that an uncondensed gas remains after the condensation. In apreferred embodiment of the invention, liquid condensate comprisingwater and substantially free of solvent monocarboxylic acid is recoveredby condensation from the pressurized gas by indirect heat exchange togenerate steam or another heated heat exchange fluid useful in othersteps or processes. An exhaust gas remaining after condensation is underpressure and typically comprises incondensable components of theoxidation reaction off-gas. It may also include minor amounts ofaromatic feed material and solvent by-products generated in liquid phaseoxidation that pass into the high pressure oxidation overhead. Condenserexhaust gas, while normally under pressure lower than the liquid phaseoxidation off-gas, nonetheless is under high pressure and hassubstantial energy content. Accordingly, the invention in someembodiments provides for recovery of energy from pressurized condenserexhaust gas. Energy can be recovered as heat, work, or a combinationthereof.

In some embodiments, the invention also can provide for improvedrecoveries and re-use of monocarboxylic acid solvent for liquid phaseoxidation. In addition to substantial separation of a solvent-richliquid from oxidation reaction off-gas device and suitability of thesolvent-rich liquid for return to or use in oxidation, the inventedprocess includes embodiments that comprise directing to separation asreflux a liquid comprising a purification mother liquor remaining afterrecovery of a purified aromatic carboxylic acid product from apurification reaction solution. In such embodiments, not only oxidationby-products, for example, carboxybenzaldehyde and toluic acid oxidationintermediates convertible terephthalic or isophthalic acid as desiredaromatic acid products, but also solvent monocarboxylic acid, such assolvent residues in the impure aromatic carboxylic acid products used toform purification solutions, or minor amounts of solvent remaining inthe liquid condensate comprising water condensed from the pressurizedgas from separation, can be returned to oxidation.

Recoveries of solvent monocarboxylic acid, reaction products thereofgenerated in liquid phase oxidation, unreacted aromatic feed materialfrom the oxidation or combinations thereof present in high pressurevapor phase from oxidation and carried over to a pressurized gasremaining after substantial separation of solvent monocarboxylic acidand water in the vapor phase are further enhanced according to otherembodiments in which the pressurized gas is condensed to recover liquidcomprising water while leaving a high pressure condenser exhaust gascooled to a temperature at which one or more scrubbing agents iseffective for removing one or more of the feed material, solvent andoxidation by-products of the solvent. The resulting gas can be furthertreated for separation of feed material and/or such solvent by-productsand, in a further embodiment, a stream comprising feed materials,solvent by-products or combinations thereof can be directed to liquidphase oxidation.

In one aspect, the invention provides an apparatus for manufacture ofaromatic carboxylic acids. The apparatus affords improved capabilitiesfor recovery of energy and for avoiding materials losses in processoperation. In some of its embodiments, the apparatus is configured toprovide added benefit by reducing corrosivity of process gas streams,such that components of the apparatus and, in some cases, of auxiliaryor other process equipment can be constructed of metals and alloys withmoderate corrosion resistance, such as stainless steels, mild steels orduplex steels, as alternatives to titanium, nickel alloy steels andother more expensive, highly corrosion resistant metals conventionallyused in aromatic carboxylic acid manufacture.

Briefly, the apparatus according to this aspect of the inventioncomprises a reaction vessel having a vent for removing reactor overheadvapor; a separation device capable of substantially separating a C₁₋₈monocarboxylic acid gas and water vapors in a high pressure gaseousmixture comprising the monocarboxylic acid and water and in fluidcommunication with the reaction vessel so as to receive a high pressurevapor phase removed from the reaction vessel; condensing means in fluidcommunication with the separation device adapted to extract energy froma high pressure gas by condensing at least a portion of the highpressure gas and exchanging heat with a heat sink material; and meansfor directing a condensate liquid condensed from the high pressure gasto at least one vessel of an apparatus for purifying an aromaticcarboxylic acid. A preferred apparatus further comprises an expander influid communication with the condensing means. A preferred separationdevice comprises one or more high pressure distillation columns.Preferably, the condenser is adapted to condense as little as about 20%to about 60% to all or substantially all, of the water present in a highpressure gas stream introduced to the condenser. Optionally, thecondenser can be further adapted to return a portion of condensateliquid condensed from a high pressure overhead gas stream to separation.

In somewhat greater detail, the apparatus according to this aspect ofthe invention comprises a reaction vessel rated for a first pressure andsuitable for liquid phase oxidation of an aromatic feed material withgaseous oxygen in a liquid phase reaction mixture comprisingmonocarboxylic acid solvent and water under conditions effective tomaintain a liquid phase reaction mixture and generate a high pressurevapor phase and comprising at least one vent for removing a highpressure vapor phase from the vessel; a separation device rated for asecond pressure which is not substantially less than the first pressureand comprising at least one gas inlet in flow communication with thereaction vessel for receiving a high pressure vapor phase removed fromat least one vent of the reaction vessel, at least liquid inlet forintroducing reflux liquid to the device, at least one gas outlet forremoving pressurized gas from the device, at least one liquid outlet forremoving a liquid stream from the device and a fractionating zonedisposed at a position intermediate at least one gas inlet and at leastone gas outlet and capable of substantially separating solventmonocarboxylic acid and water in the high pressure vapor phase gasreceived in the device such that a liquid stream comprising solventmonocarboxylic acid and substantially free of water and a high pressuregas comprising water and free of substantial solvent monocarboxylic acidare formed; condensing means comprising at least one gas inlet forreceiving a high pressure gas removed from at least one gas outlet ofthe separation device heat exchange means for transfer of heat from ahigh pressure gas in the condensing means to the heat exchange fluidsuch that a liquid condensate is condensed from the high pressure gasand a heat exchange fluid at an increased temperature or pressure isformed, at least one outlet for removing from the condensing means ahigh pressure exhaust gas, and at least one outlet for removingcondensate liquid from the condensing means; and means for directingcondensate liquid removed from at least one outlet of the condensingmeans to at least one vessel of an aromatic carboxylic acid purificationapparatus. Such and apparatus is preferably adapted for use to carry outpurification processes comprising contacting a solution comprisingaromatic carboxylic acid and impurities dissolved in a aqueous liquidwith hydrogen in the presence of a hydrogenation catalyst at elevatedtemperature and pressure to form a purification liquid reaction mixtureand recovering a solid aromatic carboxylic acid product with reducedimpurities from the purification reaction mixture. A preferred apparatusfor manufacture of a purified aromatic carboxylic acid by such a processcomprises at least one reaction vessel adapted for contacting a liquidpurification reaction solution with hydrogen at elevated temperature andpressure in the presence of a hydrogenation catalyst to form apurification liquid reaction mixture and, more preferably, at least oneproduct recovery vessel in flow communication with the reaction vesselfor receiving purification liquid reaction mixture removed from thereaction vessel and recovering therefrom solid aromatic carboxylic acidproduct with reduced levels of impurities. Preferably, such an apparatusalso includes one or more additional vessels such as for dissolvingcrude or impure aromatic carboxylic acid in a purification reactionsolvent, filtration or other separation of a solid purified aromaticcarboxylic acid from a liquid medium and washing of solid purifiedaromatic carboxylic acid product.

Apparatus according to an embodiment of this aspect of the invention canalso include a power recovery device in fluid communication with thecondensing means so as to receive a gas under pressure that exits thecondenser through at least one gas outlet. The power recovery devicecomprises at least one inlet for receiving a gas under pressure andmeans for extracting work from the high pressure gas.

Another aspect of the invention provides a process for manufacture ofaromatic carboxylic acids comprising contacting a feed materialcomprising at least one aromatic hydrocarbon precursor to the acid withgaseous oxygen in a liquid phase oxidation reaction mixture comprisingmonocarboxylic acid solvent and water and in the presence of a catalystcomposition comprising at least one heavy metal component in a reactionzone at elevated temperature and pressure effective to maintain a liquidphase oxidation reaction mixture and form an aromatic carboxylic acidand impurities comprising oxidation by-products of the aromatichydrocarbon precursor dissolved or suspended in the liquid phaseoxidation reaction mixture and a high pressure vapor phase comprisingsolvent monocarboxylic acid, water and minor amounts of the aromatichydrocarbon precursor and by-products; transferring a high pressurevapor phase removed from the reaction zone to a separation zone suppliedwith liquid reflux comprising water and capable of substantiallyseparating solvent monocarboxylic acid and water in the high pressurevapor phase to form a solvent monocarboxylic acid-rich, water leanliquid and a high pressure gas comprising water vapor; transferring ahigh pressure gas comprising water vapor removed from the separationzone to a condensing zone and condensing the high pressure gas to form acondensate liquid comprising water and substantially free of organicimpurities and a condensing zone exhaust gas under pressure comprisingincondensable components of the high pressure gas transferred to thecondensing zone; recovering from the condensing zone condensate liquidthat comprises water substantially free of organic impurities and issuitable without additional treatment for use as at least one liquidcomprising water in a process for purification of aromatic carboxylicacids; and directing condensate liquid comprising water substantiallyfree of organic impurities recovered from the condensing zone to aprocess for purification of aromatic carboxylic acid, at least one stepof which comprises (a) forming a purification reaction solutioncomprising aromatic carboxylic acid and impurities dissolved of slurriedin a liquid comprising water; (b) contacting a purification reactionsolution comprising aromatic carboxylic acid and impurities in a liquidcomprising water at elevated temperature and pressure with hydrogen inthe presence of a hydrogenation catalyst to form a purification liquidreaction mixture; (c) recovering solid purified product comprisingaromatic carboxylic acid with reduced levels of impurities from apurification liquid reaction mixture comprising the aromatic carboxylicacid and impurities in a liquid comprising water; and (d) washing withat least one liquid comprising water a solid purified aromaticcarboxylic acid product recovered from a purification liquid reactionmixture comprising the aromatic carboxylic acid, impurities and a liquidcomprising water; such that a liquid comprising water in at least onestep of the purification process comprises the condensate liquidcomprising water substantially free of organic impurities.

In another embodiment, a process for manufacture of aromatic carboxylicacid according to the invention comprises, in steps, at least one liquidphase oxidation step comprising contacting a feed material comprising atleast one substituted aromatic hydrocarbon in which the substituents areoxidizable to carboxylic acid groups with gaseous oxygen in a liquidphase oxidation reaction mixture comprising monocarboxylic acid solventand water and in the presence of a catalyst composition comprising atleast one heavy metal component in a reaction zone at elevatedtemperature and pressure effective to maintain a liquid phase oxidationreaction mixture and form an aromatic carboxylic acid and impuritiescomprising reaction by-products dissolved or suspended in the liquidphase oxidation reaction mixture and a high pressure vapor phasecomprising water, monocarboxylic acid, unreacted substituted aromatichydrocarbon, oxygen and reaction by-products; and at least onepurification step comprising contacting with hydrogen at elevatedtemperature and pressure in the presence of a catalyst comprising ahydrogenation catalyst metal a purification reaction solution comprisinga liquid that comprises water and has dissolved therein aromaticcarboxylic acid and impurities recovered from the liquid phase oxidationreaction mixture from at least one liquid phase oxidation step to form apurification liquid reaction mixture comprising the aromatic carboxylicacid and hydrogenated impurities dissolved in a liquid comprising water;and at least one off-gas treatment step comprising substantiallyseparating solvent monocarboxylic acid and water in a high pressurevapor phase removed from the reaction zone in at least one liquid phaseoxidation step to form a liquid comprising solvent monocarboxylic acidand a high pressure gas comprising water, unreacted feed material,reaction by-products, oxygen and a minor amount of solventmonocarboxylic acid and condensing directly from the high pressure gas acondensate liquid comprising water and substantially free of organicimpurities; and at least one step comprising directing condensate liquidcomprising water and substantially free of organic impurities condensedfrom the high pressure gas in at least one off-gas treatment step to atleast one purification step such that a liquid comprising water in thepurification step comprises the condensate liquid. In more specificembodiments, at least one purification step comprises at least one ofthe following additional steps in which a liquid comprising water isused: (a) a step comprising suspending or dissolving in a liquidcomprising water a solid product comprising aromatic carboxylic acid andimpurities recovered from the liquid reaction mixture in at least oneliquid phase oxidation step to form the purification reaction solution;(b) a step comprising forming a slurry in a liquid comprising water of asolid product comprising the aromatic carboxylic acid and reduced levelsof impurities recovered from the purification liquid reaction mixture;and (c) a step comprising washing with a liquid comprising water a solidproduct comprising the aromatic carboxylic acid with reduced levels ofimpurities recovered from the purification liquid reaction mixture.

Liquid phase oxidation, purification and off-gas treatment steps of theprocess according to embodiments of the invention preferably areintegrated such that a liquid phase oxidation product comprisingaromatic carboxylic acid and by-products and a high pressure vapor phasefrom a single liquid phase oxidation are directed to purification andoff-gas treatments, respectively, with a liquid condensed from apressurized gas from the off-gas treatment step being directed topurification for use as a liquid comprising water.

In another embodiment, a process according to the invention comprises insteps, (a) contacting a feed material comprising an aromatic hydrocarbonprecursor to the aromatic carboxylic acid and gaseous oxygen in a liquidphase oxidation reaction mixture comprising monocarboxylic acid solventand water and in the presence of a catalyst composition comprising aheavy metal component in a reaction zone at elevated temperature andpressure effective to maintain a liquid oxidation reaction mixture andto form an aromatic carboxylic acid and impurities comprising reactionby-products dissolved or suspended in the liquid phase oxidationreaction mixture and a high pressure vapor phase that comprisesmonocarboxylic acid, water, unreacted aromatic hydrocarbon precursor,oxygen gas, and reaction by-products; (b) recovering from the liquidphase oxidation reaction mixture a solid product comprising aromaticcarboxylic acid and impurities comprising reaction by-products; (c)suspending or dissolving solid product recovered from the liquid phaseoxidation reaction mixture comprising aromatic carboxylic acid andimpurities comprising reaction by-products in a liquid comprising water,at least a portion of which comprises a condensate liquid recoveredaccording to step (i), to form a purification reaction solution; (d)contacting the purification solution at elevated temperature andpressure with hydrogen in the presence of a hydrogenation catalyst toform a purification liquid reaction mixture; (e) recovering from thepurification liquid reaction mixture a solid purified product comprisingaromatic carboxylic acid with reduced levels of impurities and a liquidpurification mother liquor comprising water and minor amounts ofoxidized aromatic hydrocarbon precursor, hydrogenated derivativesthereof or combinations thereof; (f) transferring a high pressure vaporphase from step (a) comprising solvent monocarboxylic acid, water vapor,unreacted feed material, oxygen and by-products of the liquid phaseoxidation reaction to a separation zone supplied with reflux liquid andcapable of substantially separating the monocarboxylic acid solvent andwater in the high pressure vapor phase; (g) substantially separatingsolvent monocarboxylic acid and water in the high pressure vapor phasein the separation zone at elevated temperature and pressure into liquidcomprising monocarboxylic acid solvent and lean in water and a highpressure gas substantially free of monocarboxylic acid solventcomprising water, aromatic feed material, by-products of the oxidationstep and a minor amount of monocarboxylic acid solvent; (h) transferringa high pressure gas removed from the separation zone to a condensingzone and transferring heat between the pressurized gas and a heatexchange fluid to condense from the high pressure gas a condensateliquid comprising water substantially free of organic impurities andform a high pressure condensing zone exhaust gas; and (i) directing atleast a portion of the condensate liquid condensed from the pressurizedgas in step (h) to step (c).

In more specific embodiments, a liquid stream comprising the solventmonocarboxylic acid-rich liquid from the separation zone is transferredto the reaction zone. In other embodiments, cooling of the high pressuregas transferred to the condensing zone for condensation to recovercondensate liquid comprising water substantially free of organicimpurities is accomplished by transfer of heat from the high pressuregas to a heat exchange medium to generate steam or another heated fluidunder pressure; the resulting steam or heated fluid under pressure canbe used for heating in other steps or processes. Exhaust gas from thecondensing zone after condensation to recover liquid condensate is underpressure and comprises incondensable components of the high pressurevapor phase removed from a liquid phase oxidation step and directed toseparation, and can also contain traces of gaseous solventmonocarboxylic acid, water and aliphatic alcohols and esters formed byside reactions of solvent monocarboxylic acid as a result of the liquidphase oxidation. Accordingly, in other embodiments of the invention,condensation zone exhaust gas can be treated in one or more additionalsteps for recovery of unreacted feed materials and solvent or solventby-products of the oxidation step. Alternatively or in addition, energycan be recovered from a condensing zone exhaust gas pressuresubstantially free of organic impurities, such as by heat exchange togenerate steam or another heated fluid for process or other use or byconversion to mechanical energy such as by an expander or other suitabledevice.

In still further embodiments of the process, at least a portion of thereflux provided to the separation zone for separation of water andsolvent monocarboxylic acid in the high pressure vapor phase from aliquid phase oxidation reaction comprises liquid purification motherliquor from purification. In such embodiments of the process,purification of solid product recovered from liquid phase oxidationpreferably comprises a further step comprising directing a liquidcomprising purification mother liquor to a source of liquid introducedto the separation device as reflux.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described with reference to the Drawing, in which:

FIG. 1 is a schematic view illustrating embodiments of an apparatusaccording the invention and useful in the invented processes; and

FIG. 2 is a flow diagram illustrating a process according to preferredembodiments of the invention and integration of an apparatus accordingto the invention, such as in FIG. 1, and equipment used for purificationof aromatic carboxylic acids according to embodiments of the invention.

DETAILED DESCRIPTION

Aromatic carboxylic acids for which the invention is suited includemono- and polycarboxylated species having one or more aromatic rings andwhich can be manufactured by reaction of gaseous and liquid reactants ina liquid phase system. Examples of such aromatic carboxylic acidsinclude terephthalic acid, trimesic acid, trimellitic acid, phthalicacid, isophthalic acid, benzoic acid and naphthalene dicarboxylic acids.The invention is particularly suited for manufacture of pure forms ofterephthalic acid including purified terephthalic acid and so-calledmedium purity terephthalic acids.

An oxidation step of the invented process is a liquid phase oxidationthat comprises contacting oxygen gas and a feed material comprising anaromatic hydrocarbon having substituents oxidizable to carboxylic acidgroups in a liquid phase reaction mixture comprising a monocarboxylicacid solvent and water in the presence of a catalyst compositioncomprising at least one heavy metal component. The oxidation step isconducted at elevated temperature and pressure effective to maintain aliquid phase reaction mixture and form a high temperature, high pressurevapor phase. Oxidation of the aromatic feed material in the liquid phaseoxidation step produces aromatic carboxylic acid as well as reactionby-products such as partial or intermediate oxidation products of thearomatic feed material and solvent by-products. The liquid-phaseoxidation step and associated process steps can be conducted as a batchprocess, a continuous process, or a semi-continuous process. Theoxidation step can be conducted in one or more reactors.

Suitable aromatic feed materials for the oxidation generally comprise anaromatic hydrocarbon substituted at one or more positions, normallycorresponding to the positions of the carboxylic acid groups of thearomatic carboxylic acid being prepared, with at least one group that isoxidizable to a carboxylic acid group. The oxidizable substituent orsubstituents can be alkyl groups, such as a methyl, ethyl or isopropylgroups, or groups already containing oxygen, such as a hydroxyalkyl,formyl or keto group. The substituents can be the same or different. Thearomatic portion of feedstock compounds can be a benzene nucleus or itcan be bi- or polycyclic, such as a naphthalene nucleus. The number ofoxidizable substituents on the aromatic portion of the feedstockcompound can be equal to the number of sites available on the aromaticportion, but is generally fewer than all such sites, preferably 1 toabout 4 and most preferably 2. Examples of useful feed compounds, whichcan be used alone or in combinations, include toluene, ethylbenzene andother alkyl-substituted benzenes, o-xylene, p-xylene, m-xylene,tolualdehydes, toluic acids, alkyl benzyl alcohols,1-formyl-4-methylbenzene, 1-hydroxymethyl-4-methylben-zene,methylacetophenone, 1,2,4-trimethylbenzene,1-formyl-2,4-dimethyl-benzene, 1,2,4,5-tetramethyl-benzene, alkyl-,formyl-, acyl-, and hydroxylmethyl-substituted naphthalenes, such as2,6-dimethylnaphthalene, 2,6-diethylnaphthalene,2,7-dimethylnaphthalene, 2,7-diethylnaphthalene,2-formyl-6-methylnaphthalene, 2-acyl-6-methylnaphthalene,2-methyl-6-ethylnaphthalene and partially oxidized derivatives of theforegoing.

For manufacture of aromatic carboxylic acids by oxidation of theircorrespondingly substituted aromatic hydrocarbon pre-cursors, e.g.,manufacture of benzoic acid from mono-substituted benzenes, terephthalicacid from para-disubstituted benzenes, phthalic acid fromortho-disubstituted benzenes, and 2, 6 or 2,7 naphthalene dicarboxylicacids from, respectively, 2,6- and 2,7-disubstituted naphthalenes, it ispreferred to use relatively pure feed materials, and more preferably,feed materials in which content of the pre-cursor corresponding to thedesired acid is at least about 95 wt. %, and more preferably at least 98wt. % or even higher. A preferred aromatic hydrocarbon feed for use tomanufacture terephthalic acid comprises para-xylene. A preferred feedmaterial for making benzoic acid comprises toluene.

Solvent for the liquid phase reaction of aromatic feed material toaromatic carboxylic acid product in the liquid phase oxidation stepcomprises a low molecular weight monocarboxylic acid, which ispreferably a C₁-C₈ monocarboxylic acid, for example acetic acid,propionic acid, butyric acid, valeric acid and benzoic acid. Loweraliphatic monocarboxylic acids and benzoic acid are preferred becausethey are less reactive to undesirable reaction products than highermolecular weight monocarboxylic acids under reaction conditions used infor liquid phase oxidations to aromatic carboxylic acids and can enhancecatalytic effects in the oxidation. Acetic acid is most preferred.Solvents in the form of aqueous solutions thereof, for example about 80to about 95 wt. % solutions of the acid are most commonly used incommercial operations. Ethanol and other co-solvent materials thatoxidize to monocarboxylic acids under the liquid phase oxidationreaction conditions also can be used as is or in combination withmonocarboxylic acids with good results. When using a solvent comprisinga mixture of a monocarboxylic acid and such a co-solvent, co-solventsoxidizable to the same monocarboxylic are preferably used so thatsolvent separation steps are not further complicated.

In regard to solvents for the liquid phase oxidation according to theinvention, the expression “solvent monocarboxylic acid” as used hereinin reference to a component of various gaseous or liquid streams refersto a monocarboxylic acid having the same chemical composition as themonocarboxylic acid used as solvent for the liquid phase oxidation. Suchusage also distinguishes those chemical compositions from othermonocarboxylic acids that may be present as oxidation by-products. Byway of example, when the liquid phase reaction mixture for oxidationincludes acetic acid solvent, the expression “solvent monocarboxylicacid” refers to acetic acid but not other monocarboxylic acid speciessuch as benzoic and toluic acids which are common partial orintermediate oxidation by-products of aromatic feed materials usedaccording to the invention. Also, as will be clear from context, theword “solvent” as used in the expression “solvent monocarboxylic acid”may, but does not necessarily, refer to the function of themonocarboxylic acid to which it refers. Thus, again by way of example,“solvent monocarboxylic acid” described as a component of a liquid phaseoxidation reaction mixture is present as solvent for the mixture;however, “solvent monocarboxylic acid” described as a component presentin a high pressure vapor phase generated in the oxidation or as acomponent of a liquid phase separated from such a vapor phase is notintended to denote that the monocarboxylic acid is functioning as asolvent.

Catalysts used for the liquid oxidation comprise materials that areeffective to catalyze oxidation of the aromatic feed material toaromatic carboxylic acid. Preferred catalysts are soluble in the liquidphase reaction mixture used for oxidation because soluble catalystspromote contact among catalyst, oxygen gas and liquid feed materials;however, heterogeneous catalyst or catalyst components may also be used.Typically, the catalyst comprises at least one heavy metal componentsuch as a metal with atomic weight in the range of about 23 to about178. Examples of suitable heavy metals include cobalt, manganese,vanadium, molybdenum, chromium, iron, nickel, zirconium, cerium or alanthanide metal such as hafnium. Suitable forms of these metalsinclude, for example, acetates, hydroxides, and carbonates. Preferredcatalysts comprise cobalt, manganese, combinations thereof andcombinations with one or more other metals and particularly hafnium,cerium and zirconium.

In preferred embodiments, catalyst compositions for liquid phaseoxidation also comprise a promoter, which promotes oxidation activity ofthe catalyst metal, preferably without generation of undesirable typesor levels of by-products. Promoters that are soluble in the liquidreaction mixture used in oxidation are preferred for promoting contactamong catalyst, promoter and reactants. Halogen compounds are commonlyused as a promoter, for example hydrogen halides, sodium halides,potassium halides, ammonium halides, halogen-substituted hydrocarbons,halogen-substituted carboxylic acids and other halogenated compounds.Preferred promoters comprise at least one bromine source. Suitablebromine sources include bromo-anthracenes, Br₂, HBr, NaBr, KBr, NH₄Br,benzyl-bromide, bromo acetic acid, dibromo acetic acid,tetrabromoethane, ethylene dibromide, bromoacetyl bromide andcombinations thereof. Other suitable promoters include aldehydes andketones such as acetaldehyde and methyl ethyl ketone.

Reactants for the liquid phase reaction of the oxidation step alsoinclude a gas comprising molecular oxygen. Air is conveniently used as asource of oxygen gas. Oxygen-enriched air, pure oxygen and other gaseousmixtures comprising molecular oxygen, typically at levels of at leastabout 10 vol. %, also are useful. As will be appreciated, as molecularoxygen content of the source increases, compressor requirements andhandling of inert gases in reactor off-gases are reduced. When air orother oxygen-containing gaseous mixtures are used as an oxygen sourcefor the process, the high pressure vapor phase generated by the liquidphase reaction in the oxidation step comprises nitrogen or other inertgas components of the oxygen source.

Proportions of aromatic feed material, catalyst, oxygen and solvent arenot critical to the invention and vary with factors that include choiceof reactants, solvent and catalyst compositions and intended aromaticcarboxylic acid product, details of process design and operatingfactors. Solvent to aromatic feedstock weight ratios ranging from about1:1 to about 30:1 are preferred, with about 2:1 to about 5:1 being morepreferred although higher and lower ratios, even in the range ofhundreds to one also can be used. Oxygen gas typically is used in atleast a stoichiometric amount based on aromatic feed material but not sogreat, taking into account reaction conditions, rates and organiccomponents of the high pressure vapor phase resulting from the liquidphase reaction, that a flammable mixture exists in the vapor phase. Incommercial operations using preferred aromatic feed materials, solventmonocarboxylic acid, catalyst compositions and operating conditions,oxygen gas, most commonly supplied in the form of air, is preferablysupplied to the liquid phase oxidation at a rate effective to provide atleast about 3 to about 5.6 moles molecular oxygen per mole of aromatichydrocarbon feed material. A high pressure vapor phase resulting fromliquid phase oxidation is preferably removed from the reaction at a ratesuch that oxygen content of the vapor phase in a reaction zone containsfrom about 0.5 to about 8 vol. % oxygen measured on a solvent-freebasis. Other things being equal, vapor phase oxygen contents in thelower portion of that range, for example up to about 3 vol. %, favorsmanufacture of pure forms of aromatic carboxylic acid with somewhatgreater impurities contents than at higher vapor phase oxygen contents.For manufacture of pure forms of terephthalic acid by liquid phaseoxidation of aromatic precursors such as para-xylene and purification ofthe resulting liquid phase oxidation product according to the invention,vapor phase oxygen contents of about 0.5 to about 2.5 vol. % arepreferred for making so-called medium purity products, in which levelsof impurities primarily comprising 4-carboxybenzaldehyde and p-toluicacid range from about 100 to about 1,000 ppmw. Higher vapor phase oxygencontents in the oxidation favor manufacture of products that can bepurified to more pure forms of terephthalic acid in which levels of suchimpurities are generally less than about 200 ppmw. Catalyst suitably isused in concentrations of catalyst metal, based on weight of aromatichydrocarbon feed and solvent, greater than about 100 ppmw, preferablygreater than about 500 ppmw, and less than about 10,000 ppmw, preferablyless than about 6,000 ppmw, more preferably less than about 3,000 ppmw.Preferably a halogen promoter and more preferably a promoter comprisingbromine, is present. Such a promoter is present in an amount such thatthe atom ratio of halogen to catalyst metal suitably is greater thanabout 0.1:1, preferably greater than about 0.2:1 and suitably is lessthan about 4:1, preferably less than about 3:1. The atom ratio ofhalogen to catalyst metal most preferably ranges from about 0.25:1 toabout 2:1. Other things being equal, reaction rates and consumption ofoxygen gas in liquid phase oxidation increase and levels of unreactedoxygen in the vapor phase from oxidation decrease, with increasedcatalyst concentrations in the oxidation reaction mixture, therebyaffording control and flexibility for manufacture of a range of pureforms of aromatic carboxylic acid by the invented process.

The liquid phase reaction for oxidation of aromatic feed material toproduct comprising aromatic carboxylic acid is conducted in a suitableoxidation reaction zone, which normally comprises one or more oxidationreaction vessels. Suitable oxidation reaction vessels are configured andconstructed to withstand the high temperature and pressure conditionsand corrosive liquid and vapor phase contents used and present in thereaction zone and to provide for addition and mixing of catalyst, liquidand gaseous reactants and solvent, removal of aromatic carboxylic acidproduct or a liquid comprising such product for recovery thereof, andremoval of a high pressure vapor phase generated by the liquid phasereaction for controlling heat of reaction. Reactor types which can beused include continuous stirred tank reactors and plug-flow reactors.Commonly, oxidation reactors comprise a columnar vessel, normally with acentral axis which extends vertically when the vessel is positioned forprocess use, having one or more mixing features for mixing liquidreactants and distributing oxygen gas within the liquid phase boilingreaction mixture. Typically, the mixing feature comprises one or moreimpellers mounted on a rotatable or otherwise movable shaft. Forexample, impellers may extend from a rotatable central vertical shaftReactors may be constructed of materials designed to withstand theparticular temperatures, pressures and reaction compounds used.Generally, suitable oxidation reactors are constructed using inert,corrosion-resistant materials such as titanium or with at least theirsurfaces that define interior space or volume in which liquid reactionmixture and reaction off-gas are contained lined with materials such astitanium or glass.

A reaction mixture for the liquid phase oxidation is formed by combiningcomponents comprising aromatic feed material, solvent and catalyst andadding gaseous oxygen to the mixture. In continuous or semi-continuousprocesses, components preferably are combined in one or more mixingvessels before being introduced to the oxidation zone; however, thereaction mixture can also be formed in the oxidation zone. The source ofoxygen gas can be introduced into the reactor in one or more locationsand is typically introduced in such a manner as to promote contactbetween the molecular oxygen and the other reaction compounds, forexample, by introduction of compressed air or other gaseous oxygensource into the liquid body within a lower or intermediate portion ofthe interior volume of the reaction vessel.

Oxidation of aromatic feed material to product comprising aromaticcarboxylic acid is conducted under oxidation reaction conditionseffective to maintain a liquid phase reaction mixture and form aromaticcarboxylic acid and impurities comprising by-products of the aromatichydrocarbon precursor dissolved or suspended in the liquid phasereaction mixture and generate a high temperature and pressure vaporphase, gaseous components of which are primarily solvent monocarboxylicacid (i.e., acetic acid when the oxidation reaction solvent is aceticacid) and water. The high pressure vapor phase commonly also comprisesunreacted aromatic feed material and oxygen gas that enter the vaporphase and by-products of the liquid phase reaction. When using air, ascommonly practiced in commercial scale operations, or other oxygen gassources comprising nitrogen or other inert gas components, the vaporphase will also comprise those inert components. Heat generated byoxidation is dissipated by boiling the liquid phase reaction mixture andremoving an overhead vapor phase from the reaction zone.

Generally temperatures of the liquid phase reaction are maintained atabout 120° C. or greater, and preferably at about 140° C. or greater,but less than about 250° C. and preferably less than about 230° C.Reaction temperatures in the range of about 145° C. to about 230° C. arepreferred in the manufacture of aromatic carboxylic acid products suchas terephthalic acid, benzoic acid and naphthalene dicarboxylic acid. Attemperatures lower than about 120° C., the liquid phase oxidation canproceed at rates or with conversions that are economically unattractiveor may adversely affect product quality. For example, manufacture ofterephthalic acid from para-xylene feedstock at a temperature less thanabout 120° C. can take more than 24 hours to proceed to substantialcompletion and the resulting terephthalic acid product can requireadditional processing due to its impurities content. Temperatures above250° C. are not preferred due to potential for undesirable burning andloss of solvent. Pressure of the liquid phase reaction mixture can beused to control the temperature at which the liquid phase reactionmixture boils and is selected to maintain a substantial liquid phasereaction mixture. Pressures of about 5 to about 40 kg/cm² gauge arepreferred, with preferred pressures for particular processes varyingwith feed and solvent compositions, temperatures and other factors andmore preferably ranging between about 10 to about 30 kg/cm². At areaction pressure of about 7 to about 21 kg/cm², temperature of areaction mixture comprising acetic acid as solvent, and of the vaporphase resulting from the liquid phase reaction, is about 170 to about210° C. Residence times in the reaction vessel can be varied asappropriate for given throughputs and conditions, with about 20 to about150 minutes being generally suited to a range of processes. Formanufacture of some aromatic carboxylic acids, such as manufacture ofterephthalic acid from para-xylene feed materials using acetic acidsolvent for the reaction mixture, solids contents in the boiling liquidphase reaction mixture can be as high as about 50 wt. % of the liquidreaction mixture, with levels of about 10 to about 35 wt. % being morecommon. In processes in which the aromatic acid product is substantiallysoluble in the reaction solvent, solid concentrations in the liquid bodyare negligible. As will be appreciated by persons skilled in themanufacture of aromatic carboxylic acids, preferred conditions andoperating parameters vary with different products and processes and canvary within or even beyond the ranges specified above.

Products of the liquid phase oxidation reaction include aromaticcarboxylic acid oxidized from the aromatic feed material, impuritiescomprising by-products generated as a result of the liquid phasereaction and, as noted above, a high pressure vapor phase that resultsfrom the liquid phase reaction, including boiling of the liquid phasereaction mixture to allow removal of the vapor phase for control ofreaction temperature. Specific examples of by-products of the aromaticfeed material include partial or intermediate oxidation products such astoluic acids, tolualdehydes, carboxybenzaldehydes and hydroxymethylbenzoic acids. By-products of the liquid phase reaction also includesolvent reaction products such as methanol and other lower aliphaticalcohols oxidized from the reaction solvent and esters generated byreaction of such alcohols with the solvent, examples of which includemethyl acetate, methyl propionate, methyl butyrate and the like.By-products commonly are present in both the liquid phase oxidationreaction mixture and the vapor phase resulting therefrom. Carbon oxideby-products can result from oxidation of solvent, feed materials ortheir by-products. In embodiments of the invention in which the liquidphase reaction is conducted using a source of bromine as promoter,by-products also typically include lower alkyl bromides, e.g., methylbromide when using acetic acid as the reaction solvent, which commonlyforms by reaction of bromide ions with acetic acid. As above, thesebromine-containing by-products and impurities may be present in one orboth of the liquid phase reaction mixture and the high pressure vaporphase generated therefrom. In some embodiments of the invented process,for example those in which solid product from liquid phase oxidation ispurified and a mother liquor or other recycle streams comprisingpurification step liquids or components thereof are transferred directlyor indirectly to a liquid phase oxidation, by-products such as toluicacids carried over into purification liquids as well as hydrogenatedderivatives of various by-product compounds resulting from purificationsteps also may be present in the liquid phase reaction mixture.

Water also is produced as a by-product of the liquid phase reaction inthe oxidation step. However, because water may also be present in theliquid phase reaction mixture as a result of addition thereto, forexample when using aqueous monocarboxylic acid solvents or in recyclestreams from other process steps, and also due to the significantamounts of water present in the oxidation step, whether as by-product ordeliberate addition, and inability or lack of need to distinguishbetween water of reaction and water added deliberately, the expression“by-products of the liquid phase reaction” and like expressions usedherein do not refer to water unless stated otherwise. Similarly, whenwater or water vapor is described herein as a component of variousprocess liquids, gases or streams, it is without regard to whether thewater is by-product water from liquid phase oxidation, deliberatelyadded in the process or both unless otherwise stated or clear fromcontext.

Aromatic carboxylic acid reaction product slurried or dissolved in aportion of the liquid reaction mixture from the liquid phase oxidationcan be treated using conventional techniques to recover aromaticcarboxylic acid reaction product contained therein. Typically, aromaticcarboxylic acid product and by-products of the aromatic feed material tooxidation slurried, dissolved or slurried and dissolved in liquidreaction mixture are removed from the reaction zone used for the liquidphase reaction and recovered by suitable techniques. Thus, liquid phaseoxidation according to invented process can comprise, in addition to theoxidation reaction step, a step comprising recovering from a liquidphase oxidation reaction mixture a product comprising aromaticcarboxylic acid and impurities comprising reaction by-products. Theproduct preferably is recovered as a solid product.

Soluble product dissolved in the liquid can be recovered bycrystallization, which usually is accomplished by cooling and releasingpressure on a liquid slurry or solution from the oxidation reactionzone. Solid product slurried in the liquid and solids crystallized fromreaction liquid or from crystallization solvents are convenientlyseparated from the liquids by centrifuging, filtration or combinationsthereof. Solid products recovered from the reaction liquid by suchtechniques comprise aromatic carboxylic acid and impurities comprisingby-products of the aromatic feed material. Liquid remaining afterrecovery of solid product from the liquid reaction mixture, alsoreferred to as oxidation mother liquor, comprises solvent monocarboxylicacid, water, catalyst and promoter, soluble by-products of the liquidphase oxidation and impurities that may be present such as from recyclestreams. The mother liquor normally also contains minor amounts ofaromatic carboxylic acid and partial or intermediate oxidation productsof the aromatic feed material remaining unrecovered from the liquid. Themother liquor is preferably returned at least in part to the reactionzone of at least one liquid phase oxidation so that components thereofthat are useful in the liquid phase reaction, such as catalyst,promoter, solvent and by-products convertible to the desired aromaticcarboxylic acid are re-used.

In preferred embodiments of the invention, a liquid phase reactionmixture from oxidation comprising aromatic carboxylic acid andby-products of a liquid phase oxidation reaction is recovered from theliquid by crystallization in one or more stages, such as in a singlecrystallization vessel or a series of crystallization vessels, withsequential reductions in temperature and pressure from earlier to laterstages to increase product recovery. Crystallization in two to fourstages, for example from an oxidation reaction temperature in the rangeof about 140 to about 250° C. and pressure in the range of about 5 toabout 40 kg/cm² gauge to a final crystallization temperature in therange of about 110 to about 150° C. and pressure of ambient to about 3kg/cm², provides substantial crystallization of solid aromatic acidproduct. Mother liquor separated from the solid product bycrystallization can be returned to the liquid phase reaction asdescribed above. Heat is removed from the vessels used forcrystallization by removal of a gas phase formed as a result of flashingor other pressure letdown of the reaction liquid, with a vapor phaseremoved from one or more stages preferably condensed and, directly orindirectly through one or more additional recovery stages, as discussedbelow, returned at least in part to the reaction zone for use in liquidphase oxidation.

Solid product recovered from the liquid phase oxidation, typicallycomprising aromatic carboxylic acid and impurities comprising oxidationby-products such as intermediate oxidation products of the aromatic feedmaterial, can be separated from liquid oxidation mother liquor resultingfrom recovery of the solid product by any suitable technique. Examplesinclude centrifuging, vacuum filtration, pressure filtration andfiltration using belt filters. The resulting solid product is preferablywashed after separation with liquid comprising water such as pure wateror a wash liquid comprising minor amounts of solvent monocarboxylicacid, catalyst, aromatic feedstock, oxidation by-products orcombinations thereof that can be beneficially recycled to oxidation,either directly or combined with other liquids such as oxidation motherliquor recycle or other liquids returned to the reaction zone.Separation of solid impure aromatic carboxylic acid recovered from anoxidation mother liquor and washing of solid product can be convenientlyaccomplished by solvent exchange filtration under pressure usingpressure filters such as are disclosed in U.S. Pat. No. 5,679,846, andU.S. Pat. No. 5,200,557. A preferred filtration device for suchseparations is a BHS Fest filter as described more fully in U.S. Pat.No. 5,200,557. Mother liquor and wash liquids removed from the filteredcake can be transferred directly or indirectly to liquid phaseoxidation. Filtration and washing of the solid product in multiplestages and with increasingly pure wash liquids, for example liquidsremoved from filter cake in downstream stages as wash liquid in priorstages, can provide additional benefit by concentrating solventmonocarboxylic acid displaced from filtered solids for return tooxidation. In a more specific embodiment, the filtered cake wet withwash liquid resulting from such positive displacement filtration isdirected from a final wash stage to a drying stage wherein it isoptionally contacted with inert gas, typically under light to moderatepressure, for substantial removal of residual liquid from the cake.After washing and substantial removal of wash liquid from solid productcomprising aromatic acid and by-products, the resulting solid can bedried and directed to storage or other steps, which may includepreparation of a reaction solution for purification of the solidproduct. Preferably, levels of residual solvent monocarboxylic acid insolid product directed to purification are about 5,000 parts per millionby weight (“ppmw”) or less. Solid product can be dried with a flowingstream of nitrogen or other inert gas to reduce residual solvent levels.

In addition to the aromatic carboxylic acid reaction product formed inthe liquid phase reaction of an oxidation step according to the inventedprocess, a high pressure vapor phase is generated, comprising solventmonocarboxylic acid and water and usually also comprising by-products ofthe liquid phase oxidation and unreacted aromatic feed material andoxygen gas and, if present, inert components of the oxygen source, asdescribed above. Temperature and pressure of the vapor phase present inthe reaction zone corresponds to conditions of the liquid phasereaction. An off-gas treatment step according to the invention providesfor recoveries of energy, materials and combinations thereof.

An off-gas treatment step of the invented process comprisessubstantially separating solvent monocarboxylic acid and water vaporsfrom a stream comprising a high pressure vapor phase that is removedfrom at least one liquid phase oxidation step such that at least onesolvent monocarboxylic acid-rich liquid phase and at least one highpressure gas comprising water and typically also comprising unreactedfeed material, reaction by-products, oxygen and a minor amount ofsolvent monocarboxylic acid are formed and condensing directly from thehigh pressure gas a condensate liquid comprising water and substantiallyfree of organic impurities such as solvent monocarboxylic acid,unreacted aromatic feed material to liquid phase oxidation and oxidationby-products of the feed material. Separation is conducted with the highpressure vapor phase at a temperature and under pressure notsubstantially less than temperature and pressure of the vapor phase inthe liquid phase oxidation step from which the vapor phase is removed.The off-gas treatment preferably comprises steps that comprisetransferring a high pressure vapor phase from at least one liquid phaseoxidation step as described above comprising gaseous solventmonocarboxylic acid and water, and usually also comprising unreactedfeed material, oxygen and by-products of the oxidation step to aseparation zone supplied with reflux liquid and capable of substantiallyseparating the solvent monocarboxylic acid and water in the highpressure vapor phase, substantially separating solvent monocarboxylicacid and water in the high pressure vapor phase in the separation zoneat elevated temperature and pressure into a liquid phase, which is richin solvent monocarboxylic acid, lean in water and can also comprisecomponents of the high pressure vapor phase less volatile than thesolvent monocarboxylic acid, and a high pressure gas substantiallycomprising water vapor and substantially free of solvent monocarboxylicacid and other organic impurities such as unreacted aromatic feedmaterials and its by-products from oxidation, transferring high pressuregas removed from the separation device to a condensing zone to condensefrom the high pressure gas a liquid condensate comprising watersubstantially free of organic impurities. Condensation preferably isconducted by transferring heat between the high pressure gas and a heatexchange fluid.

In greater detail, separation according to an off-gas treatment stepaccording to the invention comprises directing a high pressure vaporphase removed from the reaction vessel used for liquid phase oxidationto a separation zone that is capable of operating with the vapor phaseat high temperature and pressure to substantially separate water andsolvent monocarboxylic acid in the vapor phase. The high pressure vaporphase can be transferred from the reaction zone of a liquid phaseoxidation to the separation zone directly, as where a separation deviceis mounted directly or in close association with an oxidation reactionvessel or other reaction zone, or indirectly, for example by meanssuitable conduits, valves, pumps and the like for effecting transfer. Aminor portion of the high pressure and high temperature vapor phase fromthe liquid phase oxidation may be directed to other uses, such asgeneration of high pressure steam or heat exchange fluid. Preferably,the vapor phase transferred to the separation device remains at highenough temperature and pressure so that energy content of the vaporphase entering the separation device is at least substantially retainedand the vapor phase provides sufficient heat for separation in contactwith reflux liquid supplied to the separation zone. Most preferably,transfer of the vapor phase to the separation zone is achieved bypassage directly from the reaction zone or through suitable pressurerated piping such that temperature of the vapor phase entering theseparation zone is no more than about 10° C. cooler than the reactiontemperature in the liquid phase oxidation and pressure of the vaporphase entering the separation zone is no more than about 3 kg/cm² lessthan the pressure in the liquid phase oxidation. The separation zonealso is designed for operation at high temperature and pressure, andpreferably at temperatures and pressures not substantially less than thetemperature and pressure of the high pressure vapor phase present in thereaction zone to avoid loss of energy content of the vapor phase fromthe reaction zone. More preferably, the separation zone is designed fortreating a vapor phase under pressure of at least about 80%, morepreferably at least about 90%, and still more preferably at least about95%, of the pressure of the vapor phase in the oxidation step. Pressurerating of equipment of the separation zone preferably is at least about80%, more preferably about 90 to about 110%, of the rating of theoxidation reaction vessel or zone of the oxidation step of the inventedprocess from which the vapor phase is directed to separation.

The separation zone preferably is capable of substantially separatingsolvent monocarboxylic acid and water vapors in the high pressure vaporphase introduced to separation such that a high pressure gas resultingfrom the separation contains no more than about 10%, and more preferablyno more than about 5% of the solvent monocarboxylic acid content of thevapor phase introduced to the separation zone. More preferably, solventmonocarboxylic acid content of the high pressure gaseous effluent fromseparation is no more than about 2%, and still more preferably no morethan about 1%, of the solvent monocarboxylic acid content of the vaporphase introduced to the separation zone.

The separation zone for off-gas treatment according to the invention cancomprise any device suitable for substantially separating solventmonocarboxylic acid and water in the high temperature and pressure vaporphase removed from the liquid phase oxidation into a vapor phase flowthrough the device at high temperature and pressure to obtain a liquidphase rich in solvent monocarboxylic acid and a pressurized gascomprising water, as described above. Preferred separation devices arevarious columns or towers, often referred to as distillation columns andtowers, dehydration towers, rectifying columns, water removal columnsand high efficiency separation devices, that are designed for contactbetween gas and liquid phases flowing therethrough for mass transferbetween the phases in a plurality of theoretical equilibrium stages,also sometimes referred to as “theoretical plates,” such that the gasphase is separated or apportioned into fractions with various boilingranges such that a liquid phase rich in at least one higher boilingcomponent, such as the solvent monocarboxylic acid in the inventedprocess, condenses from the vapor phase leaving a gas substantiallydepleted of such higher boiling component an comprising one or morelower boiling species, such as the water of the oxidation vapor phase inthe invented process. Temperature of the high pressure vapor phaseremoved from oxidation normally is high enough that there is no need forreboiling capability beyond that provided by the liquid phase oxidationreaction. Countercurrent flow of gas and liquid phases, such as byintroducing a gas phase at a lower portion of the device and refluxliquid at an upper portion, is preferred for promoting contact betweengas and liquid phases in the separation device. Contact also is promotedby internal structure providing surface for gas-liquid contact.

The separation zone according to the invention can comprise a singledevice or multiple devices, such as towers, columns or other structure,in series. When using two or more devices in series, they areconfigured, and their respective inlets and outlets communicate suchthat high pressure vapor phase removed from the oxidation reactionvessel flows into vapor phase flow through the series with separationtherein of water and C₁₋₈ monocarboxylic acid in the high pressure vaporand reverse flows of liquid, including reflux and solvent monocarboxylicacid-rich liquid separated from the high pressure vapor phase, within orbetween devices such that liquid rich in solvent monocarboxylic acid butlean in water can be withdrawn, preferably from a first device in theseries and a high pressure gas from the separation comprising watervapor and substantially free of organic impurities can be removed,preferably from the last device in the series.

Vapor phase removed from the liquid phase oxidation reaction zone isdirected to the separation zone maintained under conditions such thattemperature and pressure of the vapor phase introduced to the device arenot substantially reduced relative to inlet temperature and pressure asdescribe above. Temperatures of the vapor phase in the separation zonepreferably range from about 140 to about 200° C. and more preferablyfrom about 160 to about 185° C. Pressures from about 5 to about 40kg/cm² are preferred, with about 10 to about 20 kg/cm² being morepreferred.

Reflux liquid comprising water is supplied into contact with the highpressure vapor in the to the separation zone. Any suitable source ofliquid comprising water and substantially free of impurities detrimentalto separation can be utilized. Preferred sources of reflux liquidinclude liquids condensed from high pressure gases removed fromseparation and/or condensing zones according to the invented process. Inanother preferred embodiment described more fully herein, purificationmother liquor obtained in recovery of a purified aromatic carboxylicacid product from at least one purification liquid reaction mixture isdirected to separation such that reflux to the separation comprises thepurification mother liquor. Most preferably, reflux liquid forseparation comprises such a purification mother liquor and liquidcomprising water condensed from high pressure gases removed fromseparation and/or condensing zones according to the invention, which maybe supplied to separation individually or combined in one or moreindividual streams.

Reflux liquid preferably is supplied at a rate and temperature effectiveto quench heat of the liquid phase oxidation reaction transferred to theseparation zone in the vapor phase from the oxidation. When theseparation zone is coupled to a reaction vessel from liquid phaseoxidation for substantially direct transfer of vapor phase fromoxidation to separation, the reaction vessel functions as a reboiler. Insuch embodiments, the rate at which liquid reflux is supplied to theseparation zone is conveniently expressed as weight of liquid providedto the zone relative to weight of aromatic feed material introduced tothe liquid phase oxidation. Preferably, reflux liquid provided to theseparation zone according to the invented process is at a temperature inthe range of about 120 to about 170° C. and more preferably at about 130to about 160° C. At such temperatures, liquid preferably is supplied toseparation at a rate of about 4 to about 5 weights of the liquid perweight of aromatic precursor introduced to the liquid phase oxidation.

Water and solvent monocarboxylic acid vapors contained in the highpressure vapor stream removed from a liquid phase oxidation step andintroduced into the separation zone are separated such that a solventmonocarboxylic acid-rich liquid phase which is lean in water condensesfrom the high pressure vapor stream and a high pressure gas comprisingwater and substantially depleted of solvent and higher boilingcomponents remains. The separated liquid phase preferably comprises atleast about 60 wt. % solvent monocarboxylic acid and no more than about35 wt % water. More preferably, water content of the separated liquidphase is about 15 to about 30 wt %. The liquid stream from separationtypically also contains minor amounts of heavier impurities, such asminor amounts of aromatic carboxylic acid product and partial orintermediate oxidation by-products of the aromatic feed material, suchas benzoic acid and, depending on aromatic precursor used in theoxidation, m-toluic acid and/or p-toluic acid, and may also includeother components, such as catalyst metals and hydrogenated oxidationby-products introduced from streams recycled, for example to oxidationor as reflux liquid to separation, from other process steps. Content ofsuch heavier components preferably is no more than about 1 wt. %.

The solvent monocarboxylic acid-rich liquid phase condensed from thevapor phase in the separation zone is a valuable source of solvent forliquid phase oxidation. As described above, it also may includeoxidation by-products of the aromatic feed material and other componentssuitable for being returned to oxidation and converted to the desiredaromatic carboxylic acid. Other suitable uses for the liquid condensateinclude wash liquids for rotary vacuum filters or other devices used forsolid-liquid separations of recovered solid products of a liquid phaseoxidation from oxidation mother liquors or crystallization solvents andmake up to scrubbers, such as oxidation dryer scrubbers if used in theprocess. In a preferred embodiment of the invented process, at least aportion, and, more preferably, all or substantially all of the separatedliquid phase condensed from the high pressure vapor phase introduced tothe separation zone is returned to liquid phase oxidation, eitherdirectly to a reaction vessel or to holding vessels used for supply ofmakeup solvent to a reaction zone. In such embodiments, water andsolvent monocarboxylic acid in the high pressure vapor phase introducedto the separation zone are preferably separated such that a liquid phaseresulting from the separation contains about 15 to about 30 wt. % waterand, more preferably, such that water content of the separated liquidtogether with water returned to oxidation in other liquid streams fromthe process are substantially balanced with water vapor removed fromoxidation in the high pressure overhead vapor phase and liquid waterremoved from oxidation for recovery and separation of aromaticcarboxylic acid product of the oxidation.

The high pressure gas resulting from separation comprises a substantialvolume of water and is relatively free of solvent monocarboxylic acid.Preferably the gas comprises at least about 55 vol. %, and morepreferably at least about 65 vol. %, water. Solvent monocarboxylic acidcontent of the gas is generally less than about 5 and preferably lessthan about 3 wt %. Typically, the pressurized gas also containsunreacted aromatic feed material and by-products of the liquid phaseoxidation, typically in amounts ranging up to about 1 wt %. Oxygen gascontent of the pressurized gas from separation typically ranges up toabout 4 vol. %, preferably from about 1 to about 4 vol. %. Inert gascomponents of the oxygen source, which typically include nitrogen andcarbon oxides, can constitute up to about 45 vol. % of the pressurizedgas; when using air as a gaseous oxygen source, nitrogen content of thepressurized gas typically ranges from about 30 to about 40 vol. %.

Generally, pressure of the gas resulting from the separation is up toabout 1 kg/cm² gauge less than the pressure in the liquid phaseoxidation reaction. Temperature of the high pressure gas from separationis up to about 20° C. less than the temperature of the liquid phaseoxidation reaction, and preferably about 5° C. to about 15° C. less thanthe oxidation reaction temperature. Preferably, the high pressure gasfrom the separation is at a temperature greater than about 100° C., morepreferably greater than about 120° C., and less than about 250° C., morepreferably less than about 230° C. Pressure of the pressurized gasremaining after the separation is about 4 to about 40 kg/cm² gauge.

High pressure gas removed from the separation zone after substantialseparation of water and solvent monocarboxylic acid in the high pressurevapor phase from oxidation is directed continuously to a condensing zonefor condensing from the gas a liquid condensate comprising watersubstantially free of organic impurities such as solvent monocarboxylicacid and by-products of the aromatic feed material and solvent fromoxidation. The condensing zone can comprise any means effective forcondensing water substantially free of organic impurities from the highpressure gas introduced to the condensing zone. Preferably, it includesone or more condenser or heat exchange means effective for providingindirect heat transfer between the high pressure gas and a heat sinkmaterial, and preferably a heat exchange fluid. A single device or aplurality of devices in series can be employed. Shell and tube heatexchangers and kettle type condensers are examples of preferred devices.Preferably, all or substantially all of the high pressure gas fromseparation is directed to the condensing zone to enable substantialrecovery of both energy and materials therefrom. Cooling preferably isconducted under conditions such that a condensing zone exhaust gas underpressure not substantially reduced from that of the high pressure gasintroduced to the condensing zone remains after condensing the liquidcondensate and is withdrawn from the condensing means. That pressurizedcondensing zone exhaust gas comprises incondensable components of thehigh pressure gas from the separation zone, gaseous reaction by-productsand minor amounts of aromatic feed material and most preferably is at atemperature of about 50 to about 150° C. and under pressure that is nomore than about 3 kg/cm² less than the pressure of the inlet gas to thecondensing zone. More preferably, the pressure differential between agas removed from the separation device and the condensing zone exhaustgas after condensation of liquid condensate is about 2 kg/cm² or lessand most preferably about 0.5 to about 1 kg/cm².

Cooling of high pressure gas by heat exchange with a heat sink materialin the condensing zone also serves to heat the heat sink material. Theheat sink material preferably is a heat sink fluid, and most preferablywater. When using water as the heat exchange fluid, heat exchange withthe high pressure gas from separation converts the water to steam whichcan be directed to other parts of the invented process for heating or touses outside the process. Similarly, heat exchange between thepressurized gas and liquids from other process steps can be used forheating such liquids. In a preferred embodiment of the invented process,heat exchange between the high pressure gas from the separation zoneintroduced to the condensing zone and heat exchange fluid comprisingwater is conducted in a series of heat exchangers operated atsuccessively cooler temperatures such that steam at different pressuresis generated from the heat exchange water. Steam at different pressuresis preferably directed to one or more process steps in which steam undercorresponding pressure or pressures is useful for heating, while liquidcondensate comprising water at successively lower temperatures isgenerated from the pressurized gas.

Energy can be recovered from exhaust gas from the condensing zone in theform of heat, in the form of work or as both. Recovering energy as heatfor the process can reduce consumption of fuel that would otherwise beneeded to generate heat for the process. Energy recovered as work can beconverted to electricity for use in the process, thereby reducingconsumption of electricity from external sources if used in the process.

While preferred embodiments of the invention comprise condensing all orsubstantially all of the high pressure gas transferred to the condensingzone, in some embodiments of the invention, condensation of highpressure gas removed from the separation zone is conducted by extractingheat energy from the gas such that only a portion of the water contentof the gas is condensed. The effectiveness of heat exchange as an energyrecovery method decreases as additional increments of water arecondensed. Accordingly, partial condensation may be used in embodimentsof the invention to increase total energy recovery, though usually withlower materials recovery than in other embodiments, by avoiding therange in which the efficacy of heat exchange is substantially reduced.Partial condensation allows recovery of a liquid condensate comprisingsubstantially pure water with low organic impurities content andrecovery of heat energy transferred to a heat exchange fluid on coolingof the high pressure gas to condense the liquid condensate, while alsoleaving uncondensed water in a high pressure condensing zone exhaust gasfor further energy recovery in the form of work.

In embodiments of the invention in which high pressure gas directed tothe condensing zone is subjected to condensation so that its condensablecomponents are not fully condensed, the partial condensation ispreferably conducted so that about 50 to about 85% of the water contentof the inlet high pressure gas to the condensing zone is condensed. Mostpreferably, in such embodiments, about 70 to about 80% of the watercontent of the inlet pressurized gas is removed by the condensation. Theexhaust gas in such embodiments is suitable for extraction of energy inthe form of heat, such as by heat exchange, or as work, such as with anexpander.

According to other embodiments of the invention, all or substantiallyall of a high pressure gas from separation of monocarboxylic acid andwater in the high pressure vapor phase from oxidation is condensed byheat exchange with a heat sink fluid. Condensation of all orsubstantially all of the condensable components of the high pressure gasfrom separation reduces the volumetric flow of gas remaining aftercondensation to subsequent processing steps and permits use of metalswith only low or moderate corrosion resistance, such as stainlesssteels, mild steels or duplex steels, as alternatives to more expensive,highly corrosion resistant metals or alloys in equipment for subsequentoff-gas treatment steps that may be included in the process.Substantially complete condensation of condensable components of a highpressure gas removed from separation also increases the volume of liquidcondensate comprising water substantially free of organic impuritiesgenerated according to the invented process and can facilitate enhancedrecovery of aromatic feed material and solvent monocarboxylic acid orliquid phase oxidation by-products thereof remaining in uncondensedgases remaining after condensation.

Condensation can be conducted in a single step. It also can be conductedin multiple steps in which a gas stream comprising high pressure gasremoved from a separation zone is cooled to a first temperature in afirst stage to yield a first stage condensate liquid and an uncondensedportion of the gas which is subsequently condensed at a lowertemperature in a second stage to provide a second stage condensateliquid and an uncondensed portion of the gas introduced to the secondstage, and optionally one or more additional stages in which anuncondensed portion of gas from a prior stage is condensed at a lowertemperature than in the previous stage to form a liquid condensate and aremaining uncondensed gaseous portion. Heat exchange between thepressurized gas and uncondensed portions thereof in the stagedcondensers provides heat exchange fluid at different temperatures orpressures, for example moderate and low pressure steam, which can beused for heating in other process steps or outside the process. Inpreferred embodiments of the invention, two or more levels of steam areproduced for energy recovery, which is conveniently accomplished using acondensing or other low pressure steam turbine. In such embodiments,condensate liquid removed at different temperatures can be directed toother process uses with corresponding temperatures, thereby avoidingadditional heating or cooling of the condensate portions and, in somecases, limiting buildup of certain impurities in steps to whichcondensate liquids are recycled. For example, condensate liquids removedat temperatures in the range of about 90 to about 130° C. can bedirected preferentially to use in one or more steps of a purificationprocess such as mixing impure aromatic carboxylic acid in liquidcomprising water to form a purification reaction solution or ascrystallization solvent for purified aromatic carboxylic acid products.Condensate liquids recovered at higher temperatures, for example in therange of about 130 to about 160° C., are well suited, with little or noadditional heat input, as reflux to separation as such or in combinationwith aqueous liquids from other process steps such as mother liquorremaining after recovery and/or separation of purified aromaticcarboxylic acid in a purification step. Such high temperature condensateliquids can provide additional benefit when used as reflux to separationdue to their lower content of light components, such as lower alcoholsand solvent monocarboxylic acid esters thereof that are generated assolvent by-products in liquid phase oxidation and tend to condense ingreater concentrations in lower temperature condensate liquids. Lowertemperature condensates, for example those in the range of about 60 toabout 90° C., are also well suited for hot condensate uses such as washliquids for product separations and seal flush liquids in liquid phaseoxidation, purification or both, and still cooler condensate, forexample in the range of about 40 to about 50° C., for cold condensateuses such as scrubber washes. While condensation at differenttemperatures such that condensate liquid can be directed to otherprocess uses with compatible temperatures provides options for favorableenergy management in the invented process, it will be appreciated thatliquid condensate portions or streams condensed at higher or lowertemperatures than may be needed or preferred for use in other steps canbe cooled or heated as may be desired, for example by heat exchange, foruse in such other steps.

Exhaust gas from the condensing zone is under pressure and, whilesubstantially free of water vapor according to preferred embodiments ofthe invention, can retain a portion of the water from the pressurizedgas from separation depending on the extent of condensation in thecondensation step. In addition to such water vapor as may be present inthe exhaust gas, the gas can comprise incondensable components from theliquid phase oxidation off-gas, such as unreacted oxygen from oxidation,nitrogen, carbon oxides and other inert gas components if present in theoxygen source for oxidation, and carbon oxides and minor amounts ofother oxidation by-products of the feed material and of the solventmonocarboxylic acid, unreacted feed material and traces of solventmonocarboxylic acid from the off-gas not removed in other steps. Evenwhen water in the exhaust gas is substantially completely condensed intothe liquid condensate, such that the uncondensed exhaust gas remainingafter condensation is substantially free of water, pressure of theexhaust gas is also sufficiently high and, especially when the gaseousoxygen source for liquid phase oxidation is air or another gaseousmixture with significant inert gas content such that the vapor phaseremoved from oxidation and, in turn, pressurized gases from theseparation and condensing zones contain substantial inert gas content,volume of the condensing zone exhaust gas is such that it can be auseful source for recovery of energy.

According to some embodiments of the invention, energy is recovered fromthe pressurized exhaust gas from condensation. Preferably, energy isrecovered in the form of work. In these embodiments, a pressurized gasstream comprising exhaust gas from the condensing zone is transferred,directly or indirectly, to a device for recovering energy as work. Apreferred energy recovery device is an expander or similar apparatusadapted to receive a flow of gas under pressure and equipped with bladescapable of being rotated by the flowing gas, thereby generating workuseful in other process steps or outside the process and a cooled gasunder reduced pressure. Work extracted from the pressurized gas can beused, for example, to generate electricity using a generator or foroperating a compressor used to compress air or sources of gaseous oxygenused in liquid phase oxidation or other equipment requiring mechanicalwork. Such extracted energy can be used elsewhere in the process or inother processes. Alternatively, it can be stored or delivered to anelectrical grid for transmission to other locations. Exhaust gasremaining after recovery of energy as work can be vented, preferablyafter being subjected to additional treatments, for example condensationto remove water if present in appreciable amounts in the condensing zoneexhaust gas, and caustic scrubbing to remove bromine or other compoundswhich may be undesirable for atmospheric release. If desired, energyrecovery can be conducted after scrubbing or otherwise treating the gasfor removal of corrosive components. Removal of corrosive componentsbefore recovery of energy can be beneficial in allowing internalcomponents of an expander or other power recovery device to beconstructed of less corrosion-resistant materials than might otherwisebe preferred; however, treatment for removal of such components also canreduce power recoverable from the gas.

As an alternative to recovering energy from a condensing zone highpressure exhaust gas or, more preferably, as an additional steppreceding recovery of energy as in the form of work as described above,exhaust gas from condensation can be treated for removal of organic andother combustible compounds and corrosive components. Such treatments,in some embodiments, are particularly useful for recovering minoramounts of unreacted aromatic feed material and reaction products ofsolvent monocarboxylic acid from oxidation that may remain in theexhaust gas. In embodiments of the invention in which condensation ofhigh pressure gas from separation includes one or more condensations ata temperature low enough that water in the gas is substantially, andpreferably at least about 80%, condensed and volatile impurities such aslower alcohol and ester reaction products of the solvent monocarboxylicacid are substantially retained in an uncondensed exhaust gas phase thatis cooled sufficiently, preferably to a temperature in the range ofabout 40 to about 90° C., treatment for recovery of such impurities isfacilitated because the uncondensed exhaust gas from condensation iscool enough for use of liquid scrubbing agents for recovery. In otherembodiments, treatment is beneficial to reduce or eliminate organicspecies such as such unreacted feed material and solvent by-products ifnot removed otherwise, as well as corrosive alkyl bromide reactionby-products from liquid phase oxidations in which a source of bromine isused as promoter for the liquid phase oxidation catalyst and carriedover into the high pressure vapor phase generated in the liquid phaseoxidation and, in turn, into the high pressure gas removed fromseparation and exhaust gas removed from condensation. It will beappreciated that such treatments can affect the amount of energyrecoverable from the exhaust gas after condensation. Accordingly, inembodiments of the invention in which condensing zone exhaust gas istreated before recovery of energy in the form of work, preferredtreatments are conducted without substantial loss of pressure or volumeof the gas. When condensing zone exhaust gas has appreciable watercontent it also is preferred that any such treatment be conductedwithout appreciable condensation of water from the gas or cooling tosuch an extent that recovery of energy in the form of work results insignificant condensation of water. In such embodiments, pre-heating ofthe treated gas before recovery of energy may be beneficial.

In embodiments of the invention comprising treating a pressurizedexhaust gas from condensation for removal of unreacted feed material andsolvent by-products generated in the liquid phase oxidation, such aslower alkyl esters of the solvent monocarboxylic acid, treatment isbeneficial in allowing for return of such components to oxidation.Treatment also can reduce presence of such impurities in process recyclestreams and steady state equilibrium levels thereof in overall processoperation. Uncondensed gas under pressure removed from condensation canbe contacted, preferably at a temperature of about 35 to about 60° C.,with liquid scrubbing agent to provide a scrubbed gaseous phase withreduced levels of aromatic feed material, solvent or solvent by-productsand a liquid product comprising the scrubbing agent and enriched in atleast one of unreacted aromatic feed material, solvent monocarboxylicacid or its reaction products from liquid phase oxidation such as itscorresponding alcohols and esters thereof with the solvent. The liquidproduct is preferably returned to the reaction zone in a liquid phaseoxidation step. Scrubbing can be accomplished using any suitablescrubbing device and scrubbing agents for contacting a gas streamcomprising the high pressure condensation exhaust gas to remove volatilecomponents such as unreacted feed material, solvent monocarboxylic acidand/or its by-products from oxidation from the gas into a liquid phase.High pressure absorption columns with internal structure, such as traysor packed beds, for promoting contact between gases to be scrubbed andliquid scrubbing agent are commonly utilized. Suitable scrubbing agentsare materials that are liquid at the temperature of the gas to bescrubbed and in which the materials to be recovered have substantialsolubility. Examples include lower alcohols and C₁₋₈ carboxylic acidssuch as acetic acid, propionic acid, butyric acid and the like. Apreferred liquid scrubbing agent is the monocarboxylic acid used assolvent for liquid phase oxidation and mixtures thereof with water.Suitable scrubbing agents, equipment and use thereof for recovery ofoff-gas components from liquid phase oxidation of aromatic feedmaterials to aromatic carboxylic acids are described in further detailin U.S. Pat. No. 6,143,925, which is incorporated herein by reference.

Pressurized condenser exhaust gas, with or without prior treatment asfor scrubbing of unreacted feed material or solvent by-products asdescribed above, can also be treated to remove corrosive or othercombustible materials. While any means for such removal withoutsubstantial loss of pressure and volume of the gas can be employed, thegas preferably is subjected to an oxidation process, and most preferablya catalytic oxidation process for removal of organic, combustible andcorrosive components. Such treatments generally comprise heating anuncondensed gas under pressure, and comprising exhaust gas underpressure removed from condensation or after scrubbing or othertreatment, and gaseous oxygen in a combustion zone under pressure notsubstantially less than that of the pressurized gas and at elevatedtemperature effective to oxidize organic, combustible and corrosivecomponents to a less corrosive or more environmentally compatible gascomprising carbon dioxide and water. Heating under pressure with oxygengas preferably is conducted in the presence of a suitable oxidationcatalyst disposed within the combustion zone so as not to interrupt flowof the pressurized gas therethrough. The pressurized gas can optionallybe subjected to preheating before oxidation. Preheating can beaccomplished by any suitable means such as by heat exchange, directsteam injection or other suitable means. Optionally, combustiontreatment can also include scrubbing a pressurized gas removed fromcombustion to remove acidic, inorganic materials such as bromine andhydrogen bromide which are generated by oxidation of alkyl bromidespresent in the condenser exhaust gas when a bromine source is used forliquid phase oxidation as noted above.

Catalysts for catalytic oxidation generally comprise at least onetransition group element of the Periodic Table (IUPAC). Group VIIImetals are preferred, with platinum, palladium and combinations thereofand with one or more additional or adjuvant metals being especiallypreferred. Such catalyst metals may be used in composite forms such asoxides. Typically, the catalyst metals are disposed on a support orcarrier material of lower or no catalytic activity but with sufficientstrength and stability to withstand the high temperature and pressureoxidizing environment of the combustion zone. Suitable catalyst supportmaterials include metal oxides comprising one or more metals, examplesof which include mullite, spinels, sand, silica, alumina silica alumina,titania, zirconia. Various crystalline forms of such materials can beutilized, such as alpha, gamma, delta and eta aluminas, rutile andanatase titanias. Catalyst metal loadings on support compositions aresuitably fractions to several percents by weight, with higher loadingsbeing preferred for use when treating gases with significant water vaporcontent, such as about 20 vol. % or more. Catalysts can be used in anyconvenient configuration, shape or size. For example, the catalyst canbe in the form of pellets, granules, rings, spheres, and the like andpreferably may be formed into or disposed on a rigid cellular,honeycomb, perforated or porous structural configuration to promotecontact with gases present in the combustion zone without impeding gasflow through the zone. Specific examples of catalytic oxidationcatalysts for combustion treatment of exhaust gas removed fromcondensation in off-gas treatment according to the invention compriseabout one-half to about one wt % palladium supported on an aluminamonolith support.

In embodiments of the invention in which energy in the form of work isrecovered from gas comprising exhaust gas removed from a condensingzone, and especially when such a gas comprises appreciable water, e.g.,at least about 5 vol. %, the gas can optionally be heated to guardagainst presence of liquid water in the gas directed to energy recovery.Such heating can take place before, after or in combination with othertreatments or treatment steps such as thermal or catalytic oxidations.In such embodiments, heating can be accomplished by any suitabletechnique, such as by heat exchange or direct injection of steam orother heated gas. Heating to about 200° C. or greater is effective foravoiding condensation of water, with temperatures of about 250 to about350° C. preferred.

In addition to the condensing zone exhaust gas remaining aftercondensation of high pressure gas removed from the separation zone,condensation according to an off-gas treatment step of the inventedprocess results in condensation of a liquid from the pressurized gas.The condensate liquid comprises water of substantial purity. In additionto water, the condensate contains organic impurities comprising minoramounts of solvent monocarboxylic acid and traces of low molecularweight alcohol reaction products of the solvent monocarboxylic acid,solvent monocarboxylic acid esters thereof and partial or intermediateoxidation by-products of the aromatic feed material. Water andimpurities contents of the condensate can vary somewhat depending onchoice and compositions of recycle streams in various embodiments of theinvented process. Generally, however, water content of the condensateliquid is at least about 94 wt % and preferably 96 to about 98 wt %.Solvent monocarboxylic acid content of the liquid condensate is about 5wt % or less, and preferably not more than about ½ to about 3 wt %.Impurities such as lower aliphatic alcohols and their esters of thesolvent monocarboxylic acid formed as or from oxidation reactionproducts of the solvent monocarboxylic acid and oxidation by-products ofthe aromatic feed material typically are present at levels up to about 1wt % each, and preferably not greater than about 500 ppmw.

The high water and low organic impurities levels of the liquidcondensate make the liquid suitable, even without need for additionalpurification or other treatment to reduce impurities levels, for otheruses, including not only wash liquids for solid-liquid separations andreflux or wash liquids in separation of water and solvent monocarboxylicacid in a high pressure vapor phase from liquid phase oxidation, butalso as liquid comprising water in processes for making purifiedaromatic carboxylic acids. Surprisingly, the liquid condensate issuitable, even in commercial scale processes and without additionaltreatment or purification, not only as a crystallization solvent or washliquids for recoveries and separations of pure forms of aromaticcarboxylic acid product, but even as solvent for purification reactionsolutions comprising aromatic carboxylic acid and impurities dissolvedin a liquid comprising water. Accordingly, in preferred embodiments ofthe invented process, liquid condensate condensed from pressurizedcondenser exhaust gas and comprising water and substantially free oforganic impurities is directed to an aromatic carboxylic acidpurification process or step and used as fresh or makeup solvent fordissolving crude or impure aromatic carboxylic acid product to bepurified. In such embodiments, the invented process not only reduces, orcan even eliminate, requirements for demineralized or other high puritysources of water used in known aromatic carboxylic acid purificationprocesses, but also reduces volumes of process effluent liquids thatotherwise would be treated or disposed of as liquid waste.

In embodiments of the invention comprising purification or manufactureof purified aromatic carboxylic acids, at least one purification stepcomprises contacting with hydrogen at elevated temperature and pressurein the presence of a catalyst comprising a hydrogenation catalyst metala purification reaction solution comprising a liquid that compriseswater and has dissolved therein aromatic carboxylic acid and impuritiesto form a purification liquid reaction mixture comprising the aromaticcarboxylic acid and hydrogenated impurities dissolved in a liquidcomprising water. In preferred embodiments, a purification reactionsolution is formed by dissolving in a liquid comprising water a crudesolid product recovered from liquid phase oxidation comprising aromaticcarboxylic acid and impurities comprising oxidation by-products of thearomatic feed material for the oxidation. Pure forms of aromaticcarboxylic acid product containing reduced levels of impurities can berecovered from the purification liquid reaction mixture, preferably bycrystallization, and the resulting pure form of product can be separatedfrom a liquid purification mother liquor remaining after recovery of thepure form of product and/or from one or more liquids comprising water,such as crystallization solvents and wash liquids. At least one liquidcomprising water that is used in the purification comprises condensateliquid comprising water substantially free of organic impuritiesrecovered from a condensing zone in the off-gas treatment as describedherein. According to another preferred embodiment of the invention,purification mother liquor from at least one purification is directed tooff-gas treatment where it is used as reflux or wash liquid to aseparation zone for substantial separation of solvent monocarboxylicacid and water vapors in a high pressure vapor phase removed from liquidphase oxidation.

As indicated above, aromatic carboxylic acid products obtained by liquidphase oxidation of feed materials comprising aromatic compounds withoxidizable substituents, also sometimes referred to as a crude aromaticcarboxylic acid product or crude product from liquid phase oxidation,comprise aromatic carboxylic acid and one or more oxidationintermediates or by-products. Although specific chemical compositions ofintermediates and by-products vary depending composition of theoxidation feed material, oxidation reaction conditions and otherfactors, and even for given feed materials may not be fully known, theyare known to comprise one or more aromatic carbonyl compounds, such asbenzaldehydes, carboxybenzaldehydes, fluorenones and anthraquinones,that cause or correlate with undesirable color of desired aromaticcarboxylic acid products or of polyesters made therefrom and can behydrogenated to species more soluble in aqueous solution than thearomatic carbonyl compounds and aromatic carboxylic acid or to specieswith less color or color-forming tendencies. Preferred impure aromaticcarboxylic acid products to be purified according to the invention arecrude product comprising aromatic carboxylic acid and by-productsproduced by liquid phase oxidation of aromatic feed material in a liquidphase oxidation, and most preferably continuous processes in whichliquid phase oxidation and purification steps are integrated such that acrude solid product of liquid phase oxidation is a starting material forpurification. However, it also will be appreciated that the startingmaterial for purification can be or include an impure product comprisingan aromatic carboxylic acid and aromatic carbonyl impurities asdescribed above, whether present or generated as by-products from anintegrated or non-integrated liquid phase oxidation of aromatic feedmaterial or from other processes or sources. Thus, the inventionincludes embodiments in which an impure aromatic carboxylic acid productstarting material for purification comprises aromatic carboxylic acidand at least one aromatic carbonyl impurity that forms a hydrogenated,carbonyl-substituted aromatic product with greater solubility in aqueoussolution or less color or color-forming tendencies than theunhydrogenated aromatic carbonyl impurity.

Minor amounts of solvent monocarboxylic acid, such as residual solventremaining in crude product from a liquid phase oxidation step also maybe present in the impure aromatic carboxylic acid product subjected topurification. Amounts ranging from several hundred to thousands ppmw ascommonly present in products from commercial scale liquid phaseoxidations do not adversely affect purification according to theinvented process. Most preferably, solvent monocarboxylic acid contentof an aromatic carboxylic acid product to be purified does not exceedabout 10 wt %.

In greater detail, a preferred purification step according to theinvention comprises dissolving in a liquid comprising water, at least aportion of which most preferably comprises condensate liquid condensedfrom off-gas treatment as described herein and comprising watersubstantially free of organic impurities, a solid product comprisingaromatic carboxylic acid and impurities to form a purification reactionsolution, contacting the purification solution at elevated temperatureand pressure with hydrogen in the presence of a hydrogenation catalystto form a purification liquid reaction mixture, recovering from thepurification liquid reaction mixture a solid purified product comprisingaromatic carboxylic acid with reduced levels of impurities andseparating an aqueous liquid purification mother liquor comprisingoxidation by-products, hydrogenation products thereof and combinationsthereof from the recovered solid purified product.

Hydrogenation of impure aromatic carboxylic acids to reduce impuritieslevels is conducted with the impure acid in aqueous solution. Condensateliquid condensed from the pressurized exhaust gas condensed from highpressure gas from separation in at least one off-gas treatment step asdescribed herein is a preferred solvent for the purification solution.Supply of condensate liquid directly from condensation and without addedor intermediate treatments for removal of by-products or otherimpurities is preferred in continuous and integrated process operationsto avoid costs, complexities and additional equipment for addedhandling, storage or treatment of the condensate liquid, although itwill be appreciated that such added treatments, while unnecessary torender the condensate liquid suitable as a solvent for purification, arenot precluded. Similarly, while unnecessary for obtaining a liquid ofsufficient purity for use as purification solvent according theinvention, it will be appreciated that the invention contemplates use ofliquid condensate from condensation in combination with freshdemineralized water or other purified sources of water. Preferablyliquid condensate recovered from condensation of a high pressure gasfrom separation according to the invention makes up at least about 50%of the solvent for the purification reaction solution and morepreferably about 80 to about 100%.

Concentrations in the purification solvent of impure aromatic carboxylicacid to be treated in a purification step generally are low enough thatthe impure acid is substantially dissolved and high enough for practicalprocess operations and efficient use and handling of liquid used assolvent and remaining as purification mother liquor after recovery of apure form of aromatic carboxylic acid with reduced impurities frompurification reaction mixtures. Suitably, solutions comprising about 5to about 50 parts by weight impure aromatic carboxylic acid per hundredparts by weight solution at process temperatures provide adequatesolubility for practical operations. Preferred purification reactionsolutions contain about 10 to about 40 wt %, and more preferably about20 to about 35 wt %, impure aromatic carboxylic acid at the temperaturesused for purification by catalytic hydrogenation.

Catalysts suitable for use in purification hydrogenation reactionscomprise one or more metals having catalytic activity for hydrogenationof impurities in impure aromatic carboxylic acid products, such asoxidation intermediates and by-products and/or aromatic carbonylspecies. The catalyst metal preferably is supported or carried on asupport material that is insoluble in water and unreactive with aromaticcarboxylic acids under purification process conditions. Suitablecatalyst metals are the Group VIII metals of the Periodic Table ofElements (IUPAC version), including palladium, platinum, rhodium,osmium, ruthenium, iridium, and combinations thereof. Palladium orcombinations of such metals that include palladium are most preferred.Carbons and charcoals with surface areas of several hundreds orthousands m²/g surface area and sufficient strength and attritionresistance for prolonged use under operating conditions are preferredsupports. Metal loadings are not critical but practically preferredloadings are about 0.1 wt % to about 5 wt % based on total weight of thesupport and catalyst metal or metals. Preferred catalysts for conversionof impurities present in impure aromatic carboxylic acid productscomprising crude terephthalic acid obtained by liquid phase oxidation ofa feed material comprising para-xylene contain about 0.1 to about 3 wt %and more preferably about 0.2 to about 1 wt % hydrogenation metal. Forsuch uses, the metal most preferably comprises palladium.

For practical applications, catalyst is most preferably used inparticulate form, for example as pellets, extrudate, spheres orgranules, although other solid forms also are suitable. Particle size ofthe catalyst is selected such that a bed of catalyst particles is easilymaintained in a suitable purification reactor but permits flow of thepurification reaction mixture through the bed without undesirablepressure drop. Preferred average particle sizes are such that catalystparticles pass through a 2-mesh screen but are retained on a 24-meshscreen (U.S. Sieve Series) and, more preferably, through a 4-mesh screenbut with retention on a 12-mesh and, most preferably 8-mesh, screen.

Contacting aqueous purification reaction solution with hydrogen in thepresence of catalyst for purification is conducted at elevatedtemperatures and pressures. Temperatures range from about 200 to about370° C., with about 225 to about 325° C. being preferred and about 240to about 300° C. being most preferred. Pressure is at a level sufficientto maintain a liquid phase comprising the aqueous reaction solution.Total pressure is at least equal to, and preferably exceeds, the sum ofthe partial pressures of the hydrogen gas introduced to the process andwater vapor that boils off from the aqueous reaction solution at thetemperature of operation. Preferred pressures are about 35, and morepreferably about 70, to about 105 kg/cm².

The aqueous purification reaction solution is contacted with hydrogengas under hydrogenation conditions as described above in a suitablereaction vessel capable of withstanding reaction temperatures andpressures and also the acidic nature of its liquid contents. A preferredreactor configuration is a cylindrical reactor with a substantiallycentral axis which, when the reactor is positioned for process use, isvertically disposed. Both upflow and downflow reactors can be used.Catalyst typically is present in the reactor in one or more fixed bedsof particles maintained with a mechanical support for holding thecatalyst particles in the bed while allowing relatively free passage ofreaction solution therethrough. A single catalyst bed is often preferredalthough multiple beds of the same or different catalyst or a single bedlayered with different catalysts, for example, with respect to particlesize, hydrogenation catalyst metals or metal loadings, or with catalystand other materials such as abrasives to protect the catalyst, also canbe used and may provide benefits. Mechanical supports in the form offlat mesh screens or a grid formed from appropriately spaced parallelwires are commonly employed. Other suitable catalyst retaining meansinclude, for example, a tubular Johnson screen or a perforated plate.Internal components and surfaces of the reactor and the mechanicalsupport for the catalyst bed are constructed of materials that aresuitably resistant to corrosion from contact with the acidic reactionsolution and reaction product mixture. Most suitably, supports forcatalyst beds have openings of about 1 mm or less and are constructed ofmetals such as stainless steel, titanium or Hastelloy C.

In preferred embodiments of the invention, aqueous solution of impurearomatic carboxylic acid to be purified is added to the reactor vesselat elevated temperature and pressure at a position at or near the topportion of the reactor vessel and the solution flows downwardly throughthe catalyst bed contained in the reactor vessel in the presence ofhydrogen gas, wherein impurities are reduced with hydrogen, in manycases to hydrogenated products with greater solubility in the reactionmixture than the desired aromatic carboxylic acid or with less color orcolor-forming tendencies. In such a preferred mode, a liquidpurification reaction mixture comprising aromatic carboxylic acid andhydrogenated impurities is removed from the reactor vessel at a positionat or near a lower portion or bottom of the reactor.

Reactors used for purification may be operated in several modes. In onemode, a predetermined liquid level is maintained in the reactor and, fora given reactor pressure, hydrogen is fed at a rate sufficient tomaintain the predetermined liquid level. The difference between theactual reactor pressure and the vapor pressure of the vaporizedpurification solution present in the reactor head space is the hydrogenpartial pressure in the head space. Alternatively, hydrogen can be fedmixed with inert gas such as nitrogen or water vapor, in which case thedifference between the actual reactor pressure and the vapor pressure ofthe vaporized reaction solution present is the combined partial pressureof hydrogen and the inert gas mixed therewith. In such cases hydrogenpartial pressure may be calculated from the known relative amounts ofhydrogen and inert gas present in the mixture.

In another operating mode, the reactor can be filled with the aqueousliquid reaction solution so that there is essentially no reactor vaporspace but a hydrogen bubble at the top or in the head of the reactorthat expands or contracts in size to provide volume in the reactor headso that hydrogen added to the reactor is dissolved into the incomingpurification reaction solution. In such an embodiment, the reactor isoperated as a hydraulically full system with dissolved hydrogen beingfed to the reactor by flow control. The concentration of hydrogen insolution may be modulated by adjusting the hydrogen flow rate to thereactor. If desired, a pseudo-hydrogen partial pressure value may becalculated from the solution hydrogen concentration which, in turn, maybe correlated with the hydrogen flow rate to the reactor.

When operating such that process control is effected by adjusting thehydrogen partial pressure, the hydrogen partial pressure in the reactoris preferably in the range of about one-half to about 15 kg/cm² gauge orhigher, depending on pressure rating of the reactor, impurities levelsof the impure aromatic carboxylic acid, activity and age of the catalystand other considerations known to persons skilled in the art. Inoperating modes involving directly adjusting hydrogen concentration inthe feed solution, the solution usually is less than saturated withrespect to hydrogen and the reactor itself is hydraulically full. Thus,an adjustment of the hydrogen flow rate to the reactor will result inthe desired control of hydrogen concentration in the solution.

Space velocity, expressed as weight of the impure aromatic acid in thepurification reaction solution per weight of catalyst per hour, duringhydrogenation is typically about 1 hour⁻¹ to about 25 hour⁻¹, andpreferably about 2 hours⁻¹ to about 15 hours⁻¹. Residence time of thepurification liquid stream in the catalyst bed varies depending on thespace velocity.

Pure forms of aromatic carboxylic acid product with reduced levels ofimpurities relative to the crude or other impure aromatic carboxylicacid product used for preparing the purification solution is recoveredfrom the liquid purification reaction mixture. The purification reactionmixture, comprising aqueous reaction solvent having dissolved thereinaromatic carboxylic acid and hydrogenated aromatic impurities havinggreater solubility in the aqueous reaction liquid than theirunhydrogenated precursors, is cooled to separate a pure form of solidaromatic carboxylic acid with reduced impurities from the reactionmixture, leaving a liquid purification mother liquor having hydrogenatedimpurities dissolved therein. Separation is commonly achieved by coolingto a crystallization temperature, which is sufficiently low forcrystallization of the aromatic carboxylic acid to occur, therebyproducing crystals within the liquid phase. The crystallizationtemperature is sufficiently high so that dissolved impurities and theirreduction products resulting from hydrogenation remain dissolved in theliquid phase. Crystallization temperatures generally range up to 160° C.and preferably up to about 150° C. In continuous operations, separationnormally comprises removing liquid purification reaction mixture fromthe purification reactor and crystallization of aromatic carboxylic acidin one or more crystallization vessels. When conducted in a series ofstages or separate crystallization vessels, temperatures in thedifferent stages or vessels can be the same or different and preferablydecrease from each stage or vessel to the next. Crystallizationtypically also results in flashing of liquid from the purificationliquid reaction mixture, which can be recovered by condensation andrecycled to one or more of purification, one or more upstreamcrystallization stages or, in preferred embodiments of the invention, toseparation of solvent monocarboxylic acid and water vapor in a highpressure vapor phase from liquid phase oxidation. Liquid comprisingwater, which preferably comprises condensate liquid comprising watersubstantially free of organic impurities recovered from a high pressuregas from separation of solvent monocarboxylic acid and water vapors in ahigh pressure vapor phase from liquid phase oxidation, is preferablyadded to the crystallized product recovered from purification liquidreaction mixture recovered in stagewise crystallizations either directlyor, more preferably, indirectly in one or more wash liquids for thecrystallized product.

Thereafter, crystallized, purified aromatic carboxylic acid product isseparated from the purification mother liquor, including hydrogenatedimpurities dissolved therein. Separation of the crystallized product iscommonly conducted by centrifuging or by filtration. A preferredseparation comprises pressure filtration of an aqueous slurry of pureforms of aromatic carboxylic acid and washing of filter cake resultingfrom filtration with a liquid comprising water as described in U.S. Pat.No. 5,175,355, which is incorporated herein by reference. Condensateliquid condensed from off-gas treatment as described herein is apreferred liquid comprising water for use as wash liquid for the pureform of aromatic carboxylic acid.

Purification mother liquor remaining after recovery of solid purifiedaromatic carboxylic acid from the purification reaction mixturecomprises water and hydrogenated derivatives of by-products orimpurities present in the impure aromatic carboxylic acid startingmaterial. The mother liquor commonly also includes minor amounts ofaromatic carboxylic acid that remain in solution. Such hydrogenatedderivatives include compounds suitable for conversion to aromaticcarboxylic acid by liquid phase oxidation and, accordingly, in preferredembodiments of the invention, at least a portion of such hydrogenatedderivatives are transferred directly or indirectly to a liquid phaseoxidation. Residual aromatic carboxylic acid present in the motherliquor also can be transferred directly or indirectly to liquid phaseoxidation after separation from, or more preferably, together with, suchhydrogenated derivatives. Transfer of such derivatives and aromaticcarboxylic acid to oxidation is conveniently accomplished by directingat least a portion of a purification mother liquor remaining afterseparation of a solid pure form of aromatic carboxylic acid to a liquidphase oxidation step. Water content of purification mother liquor canupset water balance in oxidation unless water from purification motherliquor directed to oxidation is accounted for in other streams that maybe returned to oxidation. Transfer of hydrogenated impurities in apurification mother liquor, alone or preferably in combination witharomatic carboxylic acid present in the mother liquor, to liquid phaseoxidation is preferably accomplished without upsetting water balance inthe oxidation. More preferably, at least a portion, and most preferablysubstantially all, of a liquid mother liquor remaining after separationof the solid purified aromatic carboxylic acid from the liquidpurification reaction mixture is transferred directly or indirectly to aseparation zone of off-gas treatment of a high pressure vapor phaseremoved from oxidation according to the invention where it is used asreflux liquid. For example, if one or more distillation columns are usedfor separation of solvent monocarboxylic acid and water in a highpressure vapor phase generated by liquid phase oxidation of aromaticfeed material, purification mother liquor can be used in whole or inpart as reflux for one or more columns. Water present in mother liquoradded as reflux is substantially vaporized, entering the vapor phase inthe tower, with water retained in the vapor phase that exits the towerbecoming part of the pressurized gas from separation. Higher boilingcomponents of the mother liquor, including hydrogenated impurities suchas liquid phase oxidation by-products of the aromatic feed material forthe oxidation and aromatic carboxylic acid if present, remainsubstantially in the liquid phase and can be returned directly orindirectly to a liquid phase oxidation reaction mixture, for example aspart of the solvent monocarboxylic acid-rich liquid phase resulting fromseparation or in a stream separately removed from separation.

Purification reactor and catalyst bed configurations and operatingdetails and crystallization and product recovery techniques andequipment useful in the process according to this invention aredescribed in further detail in U.S. Pat. No. 3,584,039, U.S. Pat. No.4,626,598, U.S. Pat. No. 4,629,715, U.S. Pat. No. 4,782,181 U.S. Pat.No. 4,892,972, U.S. Pat. No. 5,175,355, U.S. Pat. No. 5,354,898, U.S.Pat. No. 5,362,908 and U.S. Pat. No. 5,616,792 which are incorporatedherein by reference.

FIG. 1 illustrates in further detail an apparatus according to theinvention and for operation of processes for manufacture of aromaticcarboxylic acids according to the invention. The apparatus comprises areaction vessel having a vent for removing reactor overhead vapor; aseparation device capable of substantially separating C₁₋₈monocarboxylic acid and water vapors in a gaseous mixture under pressurecomprising the monocarboxylic acid and water, the separation devicebeing in fluid communication with the reaction vessel so as to receive ahigh pressure vapor phase removed from the vent in the reaction vessel;condensing means in fluid communication with the separation deviceadapted to extract energy from a high pressure gas by condensing atleast a portion of the high pressure gas and exchanging heat with a heatsink material; and means for directing liquid condensate condensed fromthe high pressure gas from the condensing means to at least one vesselof an aromatic carboxylic acid purification apparatus. The figure alsodepicts a preferred embodiment in which the apparatus also includes anexpander in fluid communication with the condensing means for recoveringenergy from an exhaust gas under pressure removed from the condensingmeans.

A preferred form of apparatus, also represented in the figure, comprisesa pressure rated reaction vessel that defines a substantially enclosedinterior volume adapted to containing a liquid reaction mixture and anoverhead vapor and comprising at least one inlet for introducing aliquid to the interior volume, at least one inlet for introducing a gascomprising oxygen and under pressure to the interior volume, at leastone liquid product outlet for removing from the interior volume aproduct comprising a liquid or slurry of solid in a liquid, and at leastone vent for removing a high pressure off-gas from the interior volume;a separation device in fluid communication with the reaction vessel forreceiving into the device a high pressure off-gas removed from at leastone vent in the reaction vessel and comprising at least columncomprising at least one vapor inlet adapted to receive a high pressureoff-gas into the column into a high pressure vapor phase flowtherethrough, at least one liquid inlet adapted to receive reflux liquidinto a refluxing liquid flow through the column countercurrent to thehigh pressure vapor flow, means within the column and positionedintermediate vapor and liquid inlets and providing surface forcontacting vapor and liquid phases within the column such that a highpressure vapor phase comprising gaseous C₁₋₈ monocarboxylic acid andwater vapor received into the column is separated into a liquid phaserich in the C₁₋₈ monocarboxylic acid but lean in water and a highpressure gas comprising water and not more than about 10% of the C₁₋₈monocarboxylic acid in the high pressure off-gas received into thecolumn, at least one liquid outlet for removing a liquid therefrom, andat least one vent located above the liquid inlet for removal from thecolumn of the high pressure gas comprising water; at least onecondensing means in fluid communication with the separation device forreceiving a high pressure gas comprising water comprising at least onegas inlet adapted to receive the high pressure gas comprising water, atleast one vent adapted for removal of a high pressure exhaust gas fromthe condensing means, heat transfer means for exchanging heat between ahigh pressure gas introduced to the condensing means and a heat transfermedium and condense from the gas a liquid comprising water; and meansfor directing liquid comprising water recovered in the condensing meansto one or more liquid receptacles in an apparatus for purifying aromaticcarboxylic acid

Referring to FIG. 1, oxidation reaction vessel 10 comprises asubstantially cylindrical shell that defines a substantially enclosedinterior volume. In use, a lower portion of the interior volume containsa liquid reaction body while an overhead reaction off-gas is containedin a portion of the interior volume above the liquid level. The interiorvolume is in communication with the exterior of the reaction vesselthrough a plurality of inlets, such as represented as at 12 a, b, c andd, through which liquid aromatic feed material, solvent and solubleforms of catalyst are introduced from liquid charge vessels (not shown)and compressed air or another source of oxygen gas is introduced from acompressor or other suitable device (not shown) via suitable transferlines (as at 13 which is in flow communication with inlet 12 a). Theinlets preferably are disposed such that liquid and gaseous componentsare introduced below the liquid level in the interior of the vessel. Thereaction vessel also includes at least one outlet, as at 14, forremoving from the interior a liquid phase reaction mixture whichincludes a crude product comprising aromatic carboxylic acid andoxidation by-products, the same typically being present in solution inthe liquid or as solid particles suspended or slurried in the liquid orboth dissolved and suspended in the liquid. Reaction vessel 10 alsocomprises at least one vent or outlet 16 for removal from the vesselinterior of a high pressure vapor phase evaporated from the liquidreaction body. Vent 16 preferably is positioned to correspond to anupper portion of the vessel when it is in position for process use.

A preferred reaction vessel design is a substantially cylindrical vesselhaving a central axis extending substantially vertically when the vesselis positioned for process use. The vessel is adapted for use with astirring mechanism, such as represented at 18 with shaft 20 having oneor more impellers 22 and 24 mounted thereon and capable of being rotatedwithin the interior of the reaction vessel to stir the liquid reactionmixture present in the vessel during process use. In preferredembodiments of the invention, at least two impellers or mixing featuresare mounted on the shaft for mixing of gaseous and liquid componentswithin the liquid reaction body without adverse settling of solids inlower portions of the vessel. Axial flow impellers, generally configuredas propellers, radial flow mixers, such as flat blade disc turbines anddisperser discs, helical ribbon mixing elements, pitched blade turbineswith blades pitches for upward or downward flow, anchor-type mixersproviding predominantly tangential flow and other configurations aresuited for mixing the liquid phase oxidation reaction system andpreferably are used in various combinations to account for greatersolids content in lower regions of the liquid reaction mixture, greatergas content in upper regions and other characteristics of the liquidphase reaction mixture that can vary throughout the liquid body. Otherdesigns are disclosed in U.S. Pat. No. 5,198,156, describing mixingelements with radially extending, rotating blades mounted on a flatrotor and having a hollow blade configuration with a discontinuousleading edge, continuous trailing edge, absence of external concavesurfaces and an open outer end and preferably used in conjunction with avertical pipe or perforated gas sparger for gas distribution, and U.S.Pat. No. 5,904,423, which describes a mixer in which stirring elementsare mounted at a downward angle on a central, rotating shaft and arewedge-shaped in the direction of movement through the liquid, withradial inner ends of the trailing edges of the blades angled outwardlyin the direction of motion of the blades, and used with features forintroducing a gas from below the stirring elements into a central cavityformed by a conical disk at an end of the shaft.

At least those portions of the reaction vessel, agitator shaft andmixing elements that contact the liquid reaction mixture and overheadgas in process use are constructed of substantially corrosion resistantmaterials. Examples include titanium metal and alloys and duplexstainless steels. Titanium metal is preferred.

The reaction vessel is in flow communication with a separation devicesuch that a high pressure overhead gas removed from the vessel throughat least one overhead gas vent, as at 16, is received into theseparation device.

The separation device is designed so that in operation it is capable ofsubstantially separating C₁₋₈ monocarboxylic acid vapor from water vaporin a high pressure reactor overhead gas introduced to the device suchthat a liquid phase rich in the monocarboxylic acid and a high pressureseparation overhead gas substantially free of the acid but rich in watervapor are formed. A pressure rated column or tower, or a series ofcolumns or towers, equipped with at least one inlet for receiving a highpressure gas, at least one inlet for introduction of reflux liquid, atleast one outlet for removing a high pressure gas from the separation,and a fractionating zone comprising internal structure providing surfaceto promote mass transfer between countercurrently flowing gas and liquidphases sufficient to provide suitable theoretical equilibrium stages forseparation of solvent monocarboxylic acid and water in the vapor phaseis a preferred separation device. Preferably, the device is designed forintroduction of an inlet gas into a lower portion of the column or towerand introduction of reflux liquid at one or more upper location relativeto the gas inlet and with an intermediately positioned fractionatingzone so that countercurrent flow therethrough results from upwardpassage of vapor phase and downward flow, under force of gravity, ofliquid supplied as reflux and condensed from the ascending vapor phase.Additional features of such a tower or column can comprise one or moreoutlet or inlet ports for removal or addition of one or more gas orliquid streams, for example, removal of a liquid rich in monocarboxylicacid separated from the vapor phase.

The separation device can also be provided with a reboiler or othersuitable means for supplemental heat input although such means are notnormally needed when a high pressure vapor phase from a liquid phaseoxidation reaction vessel is introduced substantially directly to thedevice or otherwise without appreciable cooling because the oxidationreactor effectively serves as a reboiler by reason of the exothermicnature of the oxidation reaction. In preferred embodiments, directassociation or close coupling of the oxidation reactor and separationdevice are effectuated by connection directly or by suitable pressurerated piping or other conduits between one or more vents in theoxidation reaction vessel and one or more gas inlets to a separationdevice, such that a vapor phase under liquid phase reaction conditionsis removed from the reaction vessel and introduced into the separationdevice at the same or substantially the same temperature and pressure asin the reaction zone.

Preferably, the separation device is capable of separating water andsolvent monocarboxylic acid vapors in the high pressure gas introducedto the device such that a liquid phase with at least about 60 to 85parts by weight solvent monocarboxylic acid per hundred parts by weightof the liquid and a high pressure gas containing about 45 to 65 parts byweight water per hundred parts by weight of the gas are formed. Toachieve such separation, the fractionating zone of the separation deviceis configured with a plurality of theoretical equilibrium stages such ascan be provided by internal trays, structured packing, combinations oftrays and packing beds, or other structure or combinations thereofproviding surfaces within the interior of the device for mass transferbetween gaseous and liquid phases present in the device. At least about20 theoretical equilibrium stages are preferred for such separations.Separation efficiency increases with increasing theoretical equilibriumstages, other things being equal, so there is no theoretical upper limitto the number of equilibrium stages that may be included in theseparation devices used according to the invention. However, forpractical purposes, separation such that an outlet gas from theseparation device contains 10 wt. % or less of the solventmonocarboxylic acid content of the inlet vapor phase to the device canbe accomplished with at least about 20 theoretical equilibrium stagesand degrees of separation beyond that provided by about 70 such stagesmake additional stages impractical or economically inefficient.

A preferred separation device with structured packing has at least about3 beds or zones of packing, and more preferably about 4 to about 6 suchbeds, to provide adequate surface and theoretical equilibrium stages forseparation. An example of a suitable packing material is Flexipacstructured packing, which is available from KGGP LLC in the form of thinsheets of corrugated metal arranged in a crisscrossing relationship tocreate flow channels and such that their intersections create mixingpoints for liquid and vapor phases. A preferred separation device withtrays includes 30 to about 90 trays, at least about 70% of which arepositioned between an inlet for the high pressure gas introduced to theseparation device from the reaction vessel and a reflux liquid inlet.Trays in the form or sieve or bubble cap trays are preferred andpreferably have separation efficiencies of about 30 to about 60%. Thenumber of trays for a given number of theoretical equilibrium stages canbe calculated by dividing the number of stages by efficiency of thetrays

In process use, gas and liquid phases introduced into the separationdevice and present therein are at elevated temperatures and includewater, monocarboxylic acid and other corrosive components, for example,bromine compounds and their disassociation products such as hydrogenbromide that are present in an oxidation reaction overhead gas when thecatalyst used for the oxidation includes a source of bromine. Therefore,in preferred embodiments of the invention, internal structure and otherfeatures of the separation device that contact gases and liquids duringprocess operation are constructed of suitable metals to resist corrosionand other damage due to such contact. Titanium metal is a preferredmaterial of construction for such surfaces, including trays, packings orother structure of the fractionating zone. Titanium surfaces of suchstructure may be subject to undesirable accumulation of solid depositscomprising iron oxides from impurities present in various processliquids circulated through the equipment. Processes for controllingaccumulations of iron oxide deposits or content of soluble ironimpurities in process liquids are described in commonly assigned U.S.Pat. No. 6,852,879 and US 2002/374719 which are incorporated herein byreference.

In the embodiment of the invention represented in FIG. 1, the separationdevice is a high pressure distillation column 30 having a gas inlet 34for receiving off-gas removed from oxidation reactor 10 and a pluralityof trays, individual examples of which are illustrated at 31, 33, 35 and37. The distillation column comprises at least one lower outlet, as at32, for removal of a liquid stream from the column, for example tooxidation reactor 10 through line 60, optionally by way of one or moreintermediate holding tanks or reservoirs (not shown). The column alsoincludes at least one liquid inlet, as at 36, which is disposed at anupper portion of the column relative to liquid outlet 32 and above allor most of the trays for adding reflux liquid to the tower, such as bypumping liquid from a reflux reservoir or directly from suitable vesselsused in purification steps (not shown) through line 61. The column alsoincludes at least one gas outlet 38 for removing from the column a highpressure gas from separation.

Distillation column 30 can be configured with one or more additionalliquid inlets as at 44, for introduction of reflux liquid in addition toreflux liquid added as at inlet 36. Introducing reflux liquid atlocations separated from each other by one or more, and preferably about2 to about 10 theoretical equilibrium stages, can be beneficial in thecase of reflux liquid streams containing certain oxidation by-products,and especially by-products of the aromatic hydrocarbon feed to liquidphase oxidation and/or hydrogenated derivatives thereof such as may beintroduced in reflux liquids comprising a purification mother liquor orother recycle streams. Additional reflux supplied as at inlet 44 canserve to prevent such by-products from entering the vapor phase risingabove introduction point 44, thereby diminishing their presence in thehigh pressure gas resulting from separation and removed from column 30.

The separation device of the apparatus according to this aspect of theinvention is in flow communication with condensing means. The condensingmeans is adapted to receive a gas stream comprising high pressure gasremoved from the separation device and to condense from the gas stream aliquid condensate comprising water substantially free of organicimpurities, also leaving a high pressure condenser exhaust gas thatcomprises incondensable components of the gas introduced to theseparation device and may also include water vapor if not substantiallycompletely condensed from the high pressure inlet gas to the condensingmeans. The condensing means also comprises at least one outlet forremoving condensate liquid condensed from the gas introduced thereto anduncondensed gas under pressure therefrom and indirect heat exchangemeans for transferring heat between the inlet gas and a heat exchangefluid that is introduced to the device at a lower temperature orpressure and removed at a higher temperature or pressure. The condensingmeans also includes means for directing condensate liquid topurification equipment, with such means preferably being in flowcommunication with at least one vessel or liquid receiving means in apurification process apparatus such that condensate liquid removedthrough an outlet for removal of condensate can be transferred directlyto the purification equipment. The condensing means can also includemeans for directing condensate liquid to separation for use as refluxliquid. The condensing means can comprise a single or series of heatexchange devices, such as shell and tube heat exchangers, plate andframe heat exchangers or other suitable heat exchange devices, in whichheat from the inlet gas is transferred to a heat exchange medium such aswater, steam or another heat transfer fluid, to increase the temperatureor pressure of the heat exchange fluid. Use of multiple heat exchangedevices in series can be advantageous for generating steam or other heatexchange fluids at different pressures or temperatures, and condensateliquid at different temperatures, for usages with different steampressure and liquid temperature requirements or preferences.

The condenser is constructed of metals or alloys characterized bycorrosion resistance suited to the nature of the high temperature gasstreams and cooled liquids present circulated or present therein duringprocess use. Stainless steel internal surfaces are preferred, althoughother metals and alloys are also suitable.

Referring again to FIG. 1, the separation device, as represented bydistillation column 30, is in fluid communication via vent 38 andassociated transfer lines, as at 39, with condensing means 50. In thefigure, condensing means 50 is made up of two condensers, 52 and 62 inseries, each configured with suitable gas inlets, as at 54 and 64,respectively and outlets for removing gases and liquids therefrom, as at56 and 66, respectively. Condenser effluent, including condensate liquidand uncondensed exhaust gas under pressure, is passed via conduit 67 todrum 68 for disengagement of uncondensed gas from condensate liquid.Exhaust gas is directed from drum 68 to further processing via line 69and condensate liquid is removed to line 71 for transfer to one or morevessels for purification of aromatic carboxylic acid by hydrogenation ofa purification reaction solution comprising a liquid comprising waterhaving dissolved therein aromatic carboxylic acid and by-products ofliquid phase oxidation or aromatic carbonyl impurities suitable forconversion to hydrogenated derivatives more soluble than the impuritiesin aqueous solution.

In the preferred embodiment of the invented apparatus depicted in FIG.1, the apparatus also includes power recovery device, such as expander90, for generating energy in the form of work from a gas stream underpressure comprising uncondensed pressurized exhaust gas removed from thecondensing means, as from disengagement drum 68 via line 69. The powerrecovery device preferably is a gas expander or turbine adapted forexpansion of the volume of gas introduced to the device by reducingpressure of the gas stream and translating the expansion into mechanicalwork such as by rotation of a turbine. The expander can be in directcommunication with the condensing means, as illustrated in FIG. 1.Alternatively, the condensing means can communicate indirectly with thepower recovery device such that before being introduced to the powerrecovery device the gas under pressure is transferred to one or moreintermediate means (not shown) for treating the gas without substantialloss of pressure, such as in a high pressure scrubber or absorptiontower for selective removal or recovery of traces of process chemicalsor by-products remaining in the gas, or in a thermal or catalyticoxidation system or other pollution control device to remove corrosiveor combustible components of the gas.

Liquid and gaseous streams and materials used and present in the processare typically directed and transferred through suitable transfer lines,conduits and piping constructed of appropriate materials for process useand safety. It will be understood that particular elements may bephysically juxtaposed and can, where appropriate, have flexible regions,rigid regions or both. In directing streams or compounds, interveningapparatus or optional treatments can be included. For example,appropriate pumps, valves, manifolds, gas and liquid flow meters anddistributors, sampling and sensing devices and other equipment formonitoring, controlling, adjusting and diverting pressures, flows andother operating parameters may be present.

Aspects and embodiments of the invention are illustrated further in FIG.2 and the following discussion thereof. While the figure is describedwith specific reference to manufacture of a selected aromatic carboxylicacid, terephthalic acid, by liquid phase oxidation of para-xylene as apreferred feedstock in a liquid phase reaction mixture comprising waterand acetic acid as solvent monocarboxylic acid for the oxidation,off-gas treatment for separation of acetic acid and water from a highpressure vapor phase removed from oxidation and condensation of a highpressure gas removed from the separation to recover condensate liquidcomprising water, and a purification step for hydrogenating apurification reaction solution comprising a crude terephthalic acidproduct of the liquid phase oxidation dissolved in purification reactionsolvent, it will be understood that specific embodiments, features,details and preferences are described to aid in understanding theinvention but not to limit the invention or its features in any aspector embodiment.

The process illustrated in FIG. 2 also reflects preferred embodiments ofthe invented process in which liquid phase oxidation, off-gas treatmentand purification are integrated such that crude aromatic carboxylic acidproduct from the liquid phase oxidation is directed to purification foruse to form the purification solution, a high pressure off-gas from theoxidation is directed to the off-gas treatment, and a condensate liquidfrom the off-gas treatment is directed to the purification for use assolvent for the purification solution and other uses; however, it willbe understood that the invention is not to be considered limited to theparticular integration scheme represented in the figure and that variousmultiple train, shared train and other configurations are contemplatedaccording to the invention. By way of illustrative examples, productcomprising aromatic carboxylic acid and reaction by-products frommultiple liquid phase oxidations can be directed to a singlepurification step in which liquid condensed from a high pressure gasfrom off-gas treatment of a high pressure vapor phase from one or moreof those or other liquid phase oxidations is directed for use as aliquid comprising water. As additional such examples, crude product froma single liquid phase oxidation can be purified in separate purificationtrains operated in parallel, with high pressure vapor phase from theliquid oxidation subjected to off-gas treatment for recovery ofcondensate liquid and transfer thereof to either or both suchpurification trains, or as an alternative or in addition, to a processin which impure aromatic carboxylic acid from a separate oxidation orprocess is purified in a purification process or process steps asdescribed herein.

Referring to FIG. 2, liquid para-xylene feed material comprising atleast about 99 wt % para-xylene, aqueous acetic acid solution,preferably containing about 70 to about 95 wt % acetic acid, solublecompounds of cobalt and manganese, such as their respective acetates, assources oxidation catalyst metals and of bromine, such as hydrogenbromide as promoter for the catalyst and air are continuously charged tooxidation reaction vessel 110, which is a pressure rated, continuousstirred tank reactor, through inlets, one of which is depicted forpurposes of illustration as at 112. Solvent and para-xylene feed arecharged at rates providing a solvent to feed weight ratio of abut 2:1 toabout 5:1. Cobalt and manganese sources preferably are used in amountsproviding about 100 to about 800 ppmw each based on weight ofpara-xylene feed material. Bromine preferably is used in an amount suchthat the atom ratio of bromine to catalyst metals is about 0.1:1 toabout 1.5:1.

Stirring is provided by rotation of shaft 120 driven by an externalpower source (not shown) causing impellers mounted on the shaft andlocated within the liquid body in the reactor to provide forces formixing of liquids and dispersion of gases within the liquid body andavoiding settling of solids in its lower regions. Catalyst and promoter,each preferably as a solution in acetic acid solvent, are introducedinto the liquid body in the reaction vessel. Air is supplied from belowand within the sweep path of a lower impeller at a rate effective toprovide at least about 3 moles molecular oxygen per mole of aromaticfeed material.

Para-xylene oxidizes in the stirred liquid reaction mixture in reactor110, predominantly to terephthalic acid, but also reacts to formby-products including partial and intermediate oxidation products, suchas 4-carboxybenzaldehyde, 1,4-hydroxymethyl benzoic acid and p-toluicacid, and others such as benzoic acid. Solid reaction productscomprising terephthalic acid and para-xylene oxidation by-productsprecipitate from the liquid reaction mixture, with lesser amountsthereof remaining dissolved in the liquid. Solids content of the liquidslurry typically ranges up to about 40 wt. % and preferably from about20 to about 35 wt. %. Water is also generated as a product of theoxidation. The oxidation reaction is exothermic and heat generated bythe reaction causes boiling of the liquid phase reaction mixture andformation of an overhead vapor phase comprising vaporized acetic acid,water vapor, gaseous by-products of the oxidation reaction, carbonoxides, nitrogen from the air charged to the reaction and unreactedoxygen. The interior volume of reactor 110 is maintained under pressuresufficient to maintain the liquid phase nature of the reaction mixture,preferably at about 5 to about 21 kg/cm² gauge. Overhead vapor isremoved from the reactor through vent 116. The reactor contents aremaintained at an operating temperature in the range of about 160 toabout 225° C. based on the rate of removal of the vapor phase alsotaking into account temperatures and flow rates of streams removed fromand returned to the reactor as described below.

A liquid effluent comprising solid para-xylene oxidation products,including terephthalic acid, slurried in the liquid phase reactionmixture, which also contains dissolved para-xylene, oxidationby-products and catalyst metals, is removed from reaction vessel 110through a slurry outlet as at 114 and directed in stream 115 to acrystallization zone for recovery of a solid product of the oxidationcomprising terephthalic acid and oxidation by-products of thepara-xylene feedstock.

In the embodiment of the invention illustrated in FIG. 2,crystallization is conducted in multiple stirred crystallization vessels152 and 156 in series and in flow communication for transfer of productslurry from vessel 152 to vessel 156. Cooling in the crystallizationvessels is accomplished by pressure release, with the slurry cooled invessel 152 to a temperature in the range of about 150-190° C. and thenfurther to about 110-150° C. in vessel 156. One or more of thecrystallization vessels is vented, as at 154 and 158, respectively, forremoval to heat exchange means (not shown) of vapor resulting frompressure let down and generation of steam from the flashed vapor. Vaporremoved from one or more upstream crystallization vessels, such asvessel 152, to heat exchange means (not shown) is preferably condensedand liquid condensate comprising water, acetic acid solvent and solubleproducts and by-products of the oxidation can directed to one or moredownstream crystallization vessels, as at 156, to allow for recovery ofcrystallizable components such as terephthalic acid and oxidationby-products entering and condensed from the flashed vapors from one ormore upstream vessel.

Crystallization vessel 156 is in fluid communication with a solid-liquidseparation device 190, which is adapted to receive from thecrystallization vessel a slurry of solid product comprising terephthalicacid and oxidation by-products in a mother liquor from the oxidationcomprising acetic acid and water, and to separate a crude solid productcomprising terephthalic acid and by-products from the liquid. Separationdevice 190 is a centrifuge, rotary vacuum filter or pressure filter. Inpreferred embodiments of the invention, the separation device is apressure filter adapted for solvent exchange by positive displacementunder pressure of mother liquor in a filter cake with wash liquidcomprising water. The oxidation mother liquor that results from theseparation exits separation device 190 in stream 191 for transfer tomother liquor drum 192. A major portion of the mother liquor istransferred from drum 192 to oxidation reactor 110 for return to theliquid phase oxidation reaction of acetic acid, water, catalyst andoxidation reaction by-products dissolved or present as fine solidparticles in the mother liquor. Crude solid product comprisingterephthalic acid and impurities comprising oxidation by-products of thepara-xylene feedstock is conveyed, with or without intermediate dryingand storage, from separation device 190 to purification solution make upvessel 202 in stream 197. The crude solid product is slurried in make upvessel 202 in purification reaction solvent, all or at least a portion,and preferably about 60 to about 100 wt. %, of which, comprisescondensate liquid from off-gas treatment section 300 recovered bycondensation in condensing zones 350 of pressurized gas removed fromseparation device 330 and directed in series to condensers 352 and 362.If used, make up solvent, such as fresh demineralized water or suitablerecycle streams such as liquid condensed from vapors resulting frompressure letdown in crystallization of purified terephthalic acidproduct as discussed below, can be directed to make up tank 202 fromvessel 204. Slurry temperature in the make up tank preferably is about80 to about 100° C.

Crude product is dissolved to form a purification reaction solution byheating, for example to about 260 to about 290° C. in makeup tank 202 orby passage through heat exchangers (not shown) as it is transferred topurification reactor 210. In reactor 210, the purification reactionsolution is contacted with hydrogen under pressure preferably rangingfrom about 85 to about 95 kg/cm².

A portion of the purification liquid reaction mixture is continuouslyremoved from hydrogenation reactor 210 in stream 211 to crystallizationvessel 220 where terephthalic acid and reduced levels of impurities arecrystallized from the reaction mixture by reducing pressure on theliquid. The resulting slurry of purified terephthalic acid and liquidformed in vessel 220 is directed to solid-liquid separation apparatus230 in stream line 221. Vapors resulting from pressure letdown in thecrystallization reactor can be condensed by passage to heat exchangers(not shown) for cooling and the resulting condensate liquid redirectedto the process, for example as recycle to purification feed makeup tank202, through suitable transfer lines (not shown). Purified terephthalicacid exits solid-liquid separation device 230 in stream 231. Theseparation device can be a centrifuge, rotary vacuum filter, a pressurefilter or combinations of one or more thereof. Condensate liquidrecovered in condensing means 350 can be directed to the separationdevice as wash liquid for separation to replace or reduce demineralizedwater requirements for final washing of the purified product.

Purification mother liquor from which the solid purified terephthalicacid product is separated in solid-liquid separator 230 comprises water,minor amounts of dissolved and suspended terephthalic acid andimpurities including hydrogenated oxidation by-products dissolved orsuspended in the mother liquor. According to the preferred processembodiment illustrated in FIG. 2, at least a portion, and preferably allor substantially all, of the purification mother liquor is directed instream 233 to oxidation reaction off-gas treatment system 300 where itis introduced to high pressure distillation column 330 used forseparation of water and solvent acetic acid in a high pressure vaporremoved from oxidation reactor 110. The purification mother liquordirected to column 330 is introduced to the column at an upper portionthereof, as at 336, to provide liquid reflux for separation. Transfer ofpurification mother liquor from solid-liquid separation device 230 tothe high pressure distillation column also allows for recycle ofterephthalic acid and impurities in the mother liquor to oxidationreactor 110 where they are oxidized or converted to terephthalic acid,while water content of the purification mother liquor substantiallyvaporizes in the distillation column, exiting in a pressurized gasremoved from column, without significantly impacting water balance inoxidation. Transfer of purification mother liquor from solid-liquidseparation device 230 to the distillation column also reduces the volumeof liquid effluent that needs to be directed to liquid waste treatmentand provides for return of valuable terephthalic acid to oxidation and,in turn, removal thereof for recovery in oxidation crystallizers 152 and156.

Reaction off-gas generated by the liquid phase oxidation of para-xylenefeedstock in reactor vessel 110 is removed from the reactor through vent116 and directed in stream 111 to separation device 330 which, in theembodiments represented in FIG. 2, is a high pressure distillationcolumn having a plurality of trays preferably providing about 30 toabout 50 theoretical plates and is supplied with liquid for refluxthrough liquid inlet 336. The vapor stream from oxidation is introducedto column 330 preferably at temperature and under pressure of about 150to about 225° C. and about 4 to about 21 kg/cm² gauge, respectively, andnot substantially less than in oxidation reactor 110. As describedabove, FIG. 2 illustrates a preferred embodiment of the invention inwhich a portion of the reflux liquid introduced to the column comprisespurification mother liquor from which solid purified terephthalic acidis separated in solid-liquid separation device 230. Column 330 includes,for example, 80 trays (not shown). In the embodiment depicted in theFigure, about 50 to about 80 of the trays are disposed below refluxinlet 336 for substantial separation of solvent acetic acid and water inthe high pressure off gas from oxidation introduced into the column. Asdiscussed further below, about 10 to about 30 additional trays arepositioned above reflux inlet 336 but below introduction of a secondreflux liquid 344 for washing of oxidation by-products out of the vaporphase in the column.

As depicted in FIG. 2, distillation column 330 also includes, as apreferred though optional feature, at least one additional reflux liquidinlet as at 344 for supply of a portion of the total reflux, andpreferably about 30 to about 70% of the volumetric flow, to the column.Inlet 344 is positioned so that the additional reflux liquid supplied tothe column is introduced at a location that is separated from refluxinlet 336 by trays corresponding to one or more theoretical equilibriumstages, and preferably about 2 to about 10 such stages. In the figure,inlets 336 and 344 preferably are separated by about 10 to about 30trays. Additional reflux to the column can be supplied from the same ordifferent sources as reflux supplied at inlet 336, such as mother liquorfrom recovery of solid comprising a pure form of aromatic carboxylicacid from a purification liquid reaction mixture or, as in theembodiment of FIG. 2, a portion of condensate liquid recovered bycondensation of a high pressure gas from distillation column 330 incondensing zone 350. In the configuration illustrated in FIG. 2, thereflux supplied to the column 330 at inlet 344 has trays providingequilibrium stages above it which allow washing of para-xylene oxidationby-products including hydrogenated derivatives thereof introduced inpurification mother liquor reflux, such as p-toluic acid and benzoicacid, down the tower into the oxidation reactor 110 via stream 331.

Water and solvent acetic acid vapors in the high pressure vapor phaseintroduced to the distillation column are separated, such that an aceticacid-rich, water-lean liquid phase and a separator exit gas underpressure are formed. At least 95 wt. % of the acetic acid in the highpressure vapor from oxidation is separated into the liquid phase. Theliquid phase preferably comprises about 60 to about 85 wt % acetic acidand preferably no more than about 25 wt. % water. It also comprisesminor amounts of other components less volatile than the acetic acid,such as terephthalic acid and para-xylene oxidation by-products such asp-toluic acid and benzoic acid introduced with the purification motherliquor reflux and may also include other components such as solventby-products from oxidation. A high pressure gas from the separationprimarily comprises water vapor and also contains unreacted oxygen gas,minor amounts of solvent acetic acid vapors, unreacted para-xylene,oxidation by-products, carbon oxides, and nitrogen introduced from theair used as the oxygen source for oxidation.

Liquid phase resulting from separation in distillation column 330 exitsthe column at a lower portion thereof and preferably is returneddirectly or indirectly to oxidation reactor 110, as in stream 331.Return of the liquid phase to oxidation provides make up solvent aceticacid to the oxidation reaction and can reduce feedstock loss by allowingfor conversion to desired products of by-products condensed from theoxidation vapor phase as well as those recycled from purification motherliquor reflux to the column.

High pressure gas resulting from separation of water and acetic acidvapors in distillation column 330 is removed from the column anddirected to condensing means 350, which as depicted in FIG. 2, includescondensers 352 and 362, and disengagement drum 372. Preferably,condensation is conducted such that liquid condensate water at atemperature of about 40 to about 60° C. is recovered in at least onestage. In the embodiment illustrated in the figure, condensation isconducted by indirect heat exchange in condensing means 352 with waterat a temperature of about 120 to about 170° C., with effluent from thecondenser 352 directed to condenser 362 in stream 361 for condensationusing cooling water at about 30 to about 40° C. Gas and liquid effluentfrom condenser 362 is directed in stream 363 to drum 372 in whichcondensate liquid comprising water is collected and removed in stream373 and from which a condenser exhaust gas under pressure is withdrawnas stream 375. Condensate liquid recovered from the pressurized exit gasfrom distillation column 330 by condensation in condensing means 350 isat least about 95 wt. % water and also comprises minor amount of organicimpurities. The condensate liquid is transferred in stream 373 to one ormore vessels or liquid receptacles used in or for the purificationsteps. In the embodiment illustrated in FIG. 2, at least a portion, andpreferably a substantial portion, of the condensate liquid istransferred to purification solution make up tank 202 for use in formingthe crude product slurry and purification reaction solution that isdirected to purification reactor 210. Other purification vessels andliquid receiving equipment and uses to which the condensate liquid canbe directed include crystallization vessel 220 for use as clean make-upsolvent to replace purification reaction liquid vaporized in thecrystallizer and solid liquid separation device 230 for use as washliquid or seal flush. The condensate liquid also is suitable for usesoutside purification, such as reflux to distillation column 330 and washliquid for solvent exchange filters used for separating solid productsrecovered from oxidation from oxidation mother liquor.

Water used as heat exchange fluid for condensation of pressurized gasfrom distillation column 330 is heated by heat exchange in condensingmeans 350 to form pressurized steam which can be directed to an energyrecovery device such as steam turbine 450 in the process embodimentdepicted in FIG. 2. As described above with reference to FIG. 1,condensation of pressurized gas from separation of solvent acetic acidand water in the high pressure vapor phase from oxidation can beconducted using two or more condensers in series using heat exchangefluids at successively lower temperatures. In such embodiments, steamgenerated from condensation at decreasing temperatures is underdecreasing pressures, thereby allowing for efficiencies in use of steamat the different pressures by matching with differing heat or energyinputs to operations in which steam is used.

Uncondensed exhaust gas from condensation removed in stream 375comprises incondensable components such as unconsumed oxygen fromoxidation, nitrogen from the air used as oxygen source to the oxidation,carbon oxides from such air as well as from reactions in oxidation, andtraces of unreacted para-xylene and its oxidation by-products, methylacetate and methanol, and methyl bromide formed from the brominepromoter used in oxidation. In the embodiment illustrated in the figure,the uncondensed gas is substantially free of water vapor owing tosubstantially complete condensation into the condensate liquid recoveredin the condensing means.

Uncondensed exhaust gas from condensing means 350 is under pressure ofabout 10 to about 15 kg/cm² and can be transferred directly to a powerrecovery device or to a pollution control device for removing corrosiveand combustible species in advance of power recovery. As depicted inFIG. 2, uncondensed gas is first directed to treatment to removeunreacted feed materials and traces of solvent acetic acid and/orreaction products thereof remaining in the gas. Thus, uncondensed gas istransferred in stream 375 to high pressure absorber 380 for strippingpara-xylene, acetic acid, methanol and methyl acetate withoutsubstantial loss of pressure. Absorption tower 380 is adapted forreceipt of the substantially water-depleted gas remaining aftercondensation and for separation of para-xylene, solvent acetic acid andits reaction products from oxidation from the gas by contact with one ormore liquid scrubbing agents. A preferred absorber configuration,illustrated in the figure, comprises tower 380 having a plurality ofinternally disposed trays or beds or structured packing (not shown) toprovide surface for mass transfer between gas and liquid phases. Inlets(not shown) for addition of scrubbing agent to the absorber in streams381 and 383, respectively, are disposed at one or more upper, and one ormore lower portions of the tower. The absorber also includes an uppervent 382 from which a scrubbed gas under pressure comprisingincondensable components of the inlet gas to the absorber is removed vialine 385 and a lower outlet 384 for removal of a liquid acetic acidstream into which components from the gas phase comprising one or moreof para-xylene, acetic acid, methanol and/or methyl acetate have beenscrubbed. A bottoms liquid is removed from a lower portion of the towerand can be directed to reactor 110 for reuse of recovered components.

Pressurized gas removed from condensing means 350 or, in embodiments asdepicted in FIG. 2, from the vent 382 from the high pressure absorber,can be directed to pollution control means, as at 390, for conversion oforganic components and carbon monoxide in the pressurized gas from thecondenser or the absorber to carbon dioxides and water. A preferredpollution control means is a catalytic oxidation unit adapted forreceiving the second pressurized gas, optionally heating it to promotecombustion and directing the gas into contact with a hightemperature-stable oxidation catalyst disposed on a cellular or othersubstantially porous support such that gas flow through the device issubstantially unaffected. Overhead gas from absorber 380 is directed topollution control system 390 which includes preheater 392 and catalyticoxidation unit 394. The gas is heated to about 250 to 450° C. in thepreheater and passed under pressure of about 10 to 15 kg/cm² tooxidation unit 394 where organic components and by-products are oxidizedto compounds more suited for beneficial environmental management.

An oxidized high pressure gas is directed from catalytic oxidation unit394 to expander 400 which is connected to generator 420. Energy from theoxidized high pressure gas is converted to work in the expander 400 andsuch work is converted to electrical energy by generator 420. Expandedgas exits the expander and can be released to the atmosphere, preferablyafter caustic scrubbing and/or other treatments for appropriatelymanaging such releases.

That which is claimed is:
 1. A process for manufacture of aromaticcarboxylic acid comprising (a) contacting in a reaction zone a feedmaterial comprising at least one aromatic hydrocarbon precursor to theacid with gaseous oxygen in a liquid phase reaction mixture comprisingmonocarboxylic acid solvent and water and in the presence of a catalystcomprising at least one heavy metal component to form (i) an effluentcomprising an aromatic carboxylic acid and impurities comprisingoxidation by-products of the aromatic hydrocarbon precursor dissolved orsuspended in the liquid effluent, and (ii) a high pressure vapor phasecomprising solvent monocarboxylic acid, water and minor amounts of thearomatic hydrocarbon precursor and by-products; (b) separating in aseparation zone a monocarboxylic acid solvent and water from the highpressure vapor phase from step (a) to form a solvent monocarboxylicacid-rich, water lean liquid and a high pressure gas comprising watervapor; (c) condensing in a condensing zone the high pressure gas fromstep (b) to form a condensate liquid comprising water substantially freeof organic impurities; (d) purifying the effluent from step (a) in apurification zone to form purified aromatic carboxylic acid and apurification mother liquor comprising water and impurities; (e)directing at least a portion of the purification mother liquor from step(d) to a first inlet into the separation zone as reflux; and (f)directing at least a portion of the condensate liquid from step (c) tothe separation zone at a second inlet into the separation zone asreflux; wherein the first inlet is separated from the second inlet by atleast one tray corresponding to at least one equilibrium stage.
 2. Theprocess of claim 1 wherein the aromatic carboxylic acid comprisesterephthalic acid and an aromatic hydrocarbon precursor is para-xylene.3. The process of claim 1 additionally comprising recovering energy fromthe condensing zone exhaust gas from a condensing zone.
 4. The processof claim 1 wherein energy is recovered as work.
 5. The process of claim3 wherein the high pressure gas removed from a separation zone iscondensed in a condensing zone by indirect heat exchange with a heatexchange fluid comprising water to generate steam at one or morepressures.